sql: ensure pausable portals compatible with prepared_statements_cache_size

[ZhouXing19]

In #98917 we introduced prepared_statements_cache_size, which automatically deallocate LRU prepared statements. In #99663 we (will) introduce pausable portals, which enable executing prepared stmts in an interleaving manner. We need to sort out the expected interaction of both features.

There're several options:

1. Allowing both, meaning when deallocating LRU prepared stmts, we close the associated portals.
2. Disallow setting prepared_statements_cache_size if there're any active portals, or if the session var multiple_active_portals_enabled has been set to true. And vice versa.

Jira issue: CRDB-26158

Epic CRDB-25183

[rafiss]

Is a third option feasible: what if we update the LRU logic so it does not evict a prepared statement if there is still an open portal for it? And maybe instead it could try to find the next least recently used prepared statement that does not have an open portal.

If that's not feasible, my preference is option 1.

[ZhouXing19]

I ran the following test in PG and turns out that deallocating a prepared stmt won't affect associated portals' execution. This is what we're doing already, so I'm closing this issue.

send
Query {"String": "BEGIN"}
Parse {"Name": "q1", "Query": "SELECT * FROM generate_series(1,20)"}
Parse {"Name": "q2", "Query": "SELECT * FROM generate_series(1,20)"}
Bind {"DestinationPortal": "p1", "PreparedStatement": "q1"}
Bind {"DestinationPortal": "p2", "PreparedStatement": "q2"}
Execute {"Portal": "p1", "MaxRows": 1}
Execute {"Portal": "p2", "MaxRows": 1}
Query {"String": "DEALLOCATE q1"}
Execute {"Portal": "p1", "MaxRows": 1}
Execute {"Portal": "p2", "MaxRows": 1}
Sync
----

until
ReadyForQuery
ReadyForQuery
ReadyForQuery
----
{"Type":"CommandComplete","CommandTag":"BEGIN"}
{"Type":"ReadyForQuery","TxStatus":"T"}
{"Type":"ParseComplete"}
{"Type":"ParseComplete"}
{"Type":"BindComplete"}
{"Type":"BindComplete"}
{"Type":"DataRow","Values":[{"text":"1"}]}
{"Type":"PortalSuspended"}
{"Type":"DataRow","Values":[{"text":"1"}]}
{"Type":"PortalSuspended"}
{"Type":"CommandComplete","CommandTag":"DEALLOCATE"}
{"Type":"ReadyForQuery","TxStatus":"T"}
{"Type":"DataRow","Values":[{"text":"2"}]}
{"Type":"PortalSuspended"}
{"Type":"DataRow","Values":[{"text":"2"}]}
{"Type":"PortalSuspended"}
{"Type":"ReadyForQuery","TxStatus":"T"}