There's no possibility to attach handlers to connection before it's opened

Hello!

Test scenario is:

1. make pool with the following configuration:

{
  connectionLimit: 10,
  connectTimeout: 1000
}

2. create script which acquires connection from the pool and executes smth against the DB with 1 second interval
3. after 3 seconds of running the script, cut the wire

So at the second nr 6, intervalled script will still try to open connections, as connectionLimit isn't reached yet, so pool tries to open new connection (https://github.com/sidorares/node-mysql2/blob/821b2bcdc254c483e6879b5d4afc9757c55c7ed7/lib/pool.js#L50).

while pool will get this connection only once connection is established:
https://github.com/sidorares/node-mysql2/blob/821b2bcdc254c483e6879b5d4afc9757c55c7ed7/lib/pool.js#L61

which means, that if the connectTimeout is hit, here:
https://github.com/sidorares/node-mysql2/blob/master/lib/connection.js#L120

emitted error from the connection will cause the node process to die.

proposed solution would be to move this:
https://github.com/sidorares/node-mysql2/blob/master/lib/pool.js#L61

right after connection is constructed:
https://github.com/sidorares/node-mysql2/blob/master/lib/pool.js#L53

Start in lib/pool.js at connection construction and handler registration, then inspect the timeout path in lib/connection.js. Reproduce the interrupted-wire scenario and verify that a connection timing out before pool acquisition no longer causes an uncaught process error.