Connection pooling

[swizzlr]

Dropping in a placeholder here. I'll implement one in a project and we can backport it upstream.

One thread restriction is that no two threads attempt to manipulate the same PGconn object at the same time. In particular, you cannot issue concurrent commands from different threads through the same connection object. (If you need to run concurrent commands, use multiple connections.)

https://www.postgresql.org/docs/9.6/static/libpq-threading.html