Track parameter status changes across clients

[levkk]

In transaction mode, track parameter status updates, e.g. SET statement_timeout TO '30s';, and make sure they are cleaned up before a connection is placed back into the pool. This avoids backend connection contamination, especially in cases when pg_dump resets the search_path and client connections throw "table doesn't exist" errors (very scary in production).

Track which client sets which parameters and restore them before the connection is given to the client. This allows persisting session-level state across transactions in transaction pooling mode.