Fix asyncpg introspection during connection creation

[piercefreeman]

Single asyncpg connections invoke an introspection command that can be quite costly on first run with jit enabled - which it is on modern Postgres versions. This is fine when asyncpg connects directly to postgres because it wraps introspection commands in a jit=false flag. However when run behind pgbouncer, the query multiplexing strips out session based commands that toggle state like this, so JIT remains on and the queries become slow.

We need to figure out a way to cache this data internally within the runtime so it's usable across sessions.

MagicStack/asyncpg#530