PSQLConnection recreates a new SSLContext for new connection

[fabianfett]

Currently PSQLConnection recreates a new SSLContext for every encrypted connection. This is very wasteful. The SSLContext can be cached and shared between all connections to the same remote.

For more context:
AHC: cache NIOSSLContext (saves 27k allocs per conn) #362

[fabianfett]

PostgresConnection is now initialized with a NIOSSLContext.