Is your feature request related to a problem? Please describe.
While a SQL transaction is in aborted state (i.e. refusing commands, but waiting for explicit ROLLBACK), the corresponding KV transaction is still alive and may hold intents open.

Describe the solution you'd like
Ideally the KV txn would be torn down the moment it is no longer used.

Describe alternatives you've considered
Don't do anything

Additional context
Repro:

-- session 1
create table foo(v int)
begin;
insert into foo values(1)
asdasfasfasfasf
select 1 -- current txn is aborted, commands ignored until end of txn block

-- session 2
begin;
select * from foo -- blocks forever

Jira issue: CRDB-3103