play nicely with node.js domains

Long story short. When you create a connection it's added to an active
domain. When you close a connection, it's removed from a domain. It
works well and expected in a situation not involving a connection pool.

Now, with a connection pool, create and close become acquire and
release, and a connection has a longer life which is managed by a
connection pool. Every time you acquire a connection from a connection
pool, a domain, previously assigned to it when it was created, is
out-of-date, because at the time you acquire a connection, an active
domain is different and you must re-assign it to a connection. That
means a connection pool has to manage domains for us. It's not that
difficult though.