Bug 1198773

Previously, an improvement to the connection pool such that new connections could be made concurrently, made it so that the 'init on first connect' routine of a SQLAlchemy dialect would not have been completed if concurrent routines proceeded at the same time. As a result, when a SQLAlchemy engine was first used, operations which relied on the state acquired during initial startup could fail, as this information would not have been completed. To resolve this issue, with this update, 'mutexing' was added to the event system which handles the initial dialect startup phase, so that connection attempts are again serialized, but only when the engine first starts up.