Connection state is not reset on return to pool

Per comment on https://github.com/sidorares/node-mysql2/issues/925#issuecomment-471101203

This library does not reset the state of a connection returned to the pool before it is reused.

This means if any operation mutates state of the connection such as session level variables/isolation level, or begins a transaction without committing or rolling back, the new connection request is given a dirty connection object.

This can lead to rather extreme bugs.

to resolve this, the library needs to follow the industry standard practice of issuing a connection.changeUser() with the same user the connection was opened with, ensuring that the state is all reset.

I would suggest doing this on connection release rather than connection acquire so that obtaining a connection can be faster.