67866: sql: implement SQL Stats flush logic r=Azhng a=Azhng

Previous PR: #67805
Next Chained PR: #67090

## First Commit

sql: remove `count` from stmt/txn stats system table

Previously, system.statement_statistics and
system.transaction_statistics table includes a `count` column
that corresponds to `roachpb.StatementStatistics.Count` and
`roachpb.TransactionStatistics.Count` fields respectively.
The objective for that column is to make
`INSERT ON CONFLICT DO UPDATE` style query easy. However,
since early prototyping have shown that
`INSERT ON CONFLICT DO UPDATE` style statement is quite inefficient,
the SQL Stats flush mechanism will be implemented using
separate queries INSERT and UPDATE statements.
This column is no longer userful and it would require special handling.
Removing this column simplifies the flush logic and removes the
need for special handlings.

Release note (sql change): count column is removed from
 system.statement_statistics and system.transaction_statistics
 tables.

## Second Commit

sql: implement persistedsqlstats flush logic

This commit implements the initial flush logic of the
persisted sql stats subsystem.

Release note: None

68426: kv: assert txn unused in SetFixedTimestamp r=nvanbenschoten a=nvanbenschoten

This commit asserts that a transaction has not been used to read or to
write by the time that `SetFixedTimestamp` is called on it.

This was extracted from #68194 and modified to return an error from
`SetFixedTimestamp` on misuse instead of fatal-ing. This provides a
sufficient, temporary backstop for #68216 until the conn executor logic
is fixed:

```
root@127.0.0.1:26257/movr> create table t (x int);
CREATE TABLE

root@127.0.0.1:26257/movr> insert into t values (1);
INSERT 1

root@127.0.0.1:26257/movr> select crdb_internal_mvcc_timestamp, * from t;
   crdb_internal_mvcc_timestamp  | x
---------------------------------+----
  1628094563935439000.0000000000 | 1
(1 row)

root@127.0.0.1:26257/movr> begin as of system time (1628094563935439000.0000000000-1)::string;
BEGIN

root@127.0.0.1:26257/movr  OPEN> select * from t;
  x
-----
(0 rows)

root@127.0.0.1:26257/movr  OPEN> prepare y as select * from t as of system time 1628094563935439000.0000000000;
ERROR: internal error: cannot set fixed timestamp, txn "sql txn" meta={id=e5e81c19 pri=0.01517572 epo=0 ts=1628094563.935438999,0 min=1628094563.935438999,0 seq=0} lock=false stat=PENDING rts=1628094563.935438999,0 wto=false gul=1628094563.935438999,0 already performed reads
SQLSTATE: XX000
DETAIL: stack trace:
github.com/cockroachdb/cockroach/pkg/kv/kvclient/kvcoord/txn_coord_sender.go:1016: SetFixedTimestamp()
github.com/cockroachdb/cockroach/pkg/kv/txn.go:1200: SetFixedTimestamp()
github.com/cockroachdb/cockroach/pkg/sql/conn_executor_prepare.go:278: populatePrepared()
github.com/cockroachdb/cockroach/pkg/sql/conn_executor_prepare.go:220: func1()
github.com/cockroachdb/cockroach/pkg/sql/conn_executor_prepare.go:226: prepare()
github.com/cockroachdb/cockroach/pkg/sql/conn_executor_prepare.go:112: addPreparedStmt()
github.com/cockroachdb/cockroach/pkg/sql/conn_executor_exec.go:570: execStmtInOpenState()
github.com/cockroachdb/cockroach/pkg/sql/conn_executor_exec.go:126: execStmt()
github.com/cockroachdb/cockroach/pkg/sql/conn_executor.go:1626: func1()
github.com/cockroachdb/cockroach/pkg/sql/conn_executor.go:1628: execCmd()
github.com/cockroachdb/cockroach/pkg/sql/conn_executor.go:1550: run()
github.com/cockroachdb/cockroach/pkg/sql/conn_executor.go:627: ServeConn()
github.com/cockroachdb/cockroach/pkg/sql/pgwire/conn.go:645: func1()
runtime/asm_amd64.s:1371: goexit()

HINT: You have encountered an unexpected error.

Please check the public issue tracker to check whether this problem is
already tracked. If you cannot find it there, please report the error
with details by creating a new issue.

If you would rather not post publicly, please contact us directly
using the support form.

We appreciate your feedback.

root@127.0.0.1:26257/? ERROR>
```

68442: kv: include RangeID in rangefeed goroutine stacks r=nvanbenschoten a=nvanbenschoten

This commit includes the RangeID in each of a rangefeed processor and
its registations' associated goroutine stacks. This is a cheap and easy
way to get better observability into the ranges that have active
rangefeeds. It also tells us where those goroutines are spending their
time.

This will also become easier to use in Go 1.17, which improved the
format of stack traces.

68443: parser: add VIRTUAL syntax to help r=RaduBerinde a=rafiss

Release note: None

Co-authored-by: Azhng <archer.xn@gmail.com>
Co-authored-by: Nathan VanBenschoten <nvanbenschoten@gmail.com>
Co-authored-by: Rafi Shamim <rafi@cockroachlabs.com>