Prior to this patch, CockroachDB was following the pg docs to the
letter regarding the relationship between `search_path` and the
"current schema": the current schema was taken to always be the first
item in `search_path`, as indicated in
https://www.postgresql.org/docs/10/static/functions-info.html
("current_schema returns the name of the schema that is first in the
search path (or a null value if the search path is empty).")

This in turn impacts the creation of objects (tables, views, etc) when
the name is unqualified: pg also specifies that such objects are to be
created in the current schema.

Unfortunately the pg doc is incorrect, as pg has a more nuanced
behavior: only the _valid_ schemas in `search_path` are considered to
determine the current schema in that way.

This was causing an unfortunate visible compatibility bug: if the user
specified an invalid schema in the search path, subsequent create
operations would fail with CockroachDB and succeed with pg.

This patch corrects this issue by adopting pg's behavior.

Ditto for `current_schemas`.

Release note (sql change, bug fix): the built-in functions
`current_schema` and `current_schemas` now only consider valid
schemas, like PostgreSQL does.