Implement Close for prepared statements

[levkk]

Features

Add support for Close (F). In transaction mode, the prepared statement will be removed from the client cache, but not from the server cache. In session mode, the prepared statement will be removed from both server and client caches and from the Postgres server itself.

Add prepared statement cache size limit, with a cleanup job that removes statements that are above cache size. Cost of removal is amortized across clients, not ran as a maintenance loop out of connection lifetime. We need to write our own internal pool implementation to add the ability to run maintenance on connections out of usage scope. I think the impact at the moment will be mostly negligible because cache eviction should only be active on very active systems that are constantly used, so running a cleanup job out of client usage scope will still block the connection when it's needed.

Bugs

Fixed what may have been a big latency bottleneck caused by incorrect arc_swap usage to fetch the idle in transaction config value.