Set connect_timeout on database connections.
Set connect_timeout on database connections.

Summary:
Otherwise, an attempt to instantiate a database connection may block the event
loop forever if the database server is unresponsive.

Test Plan:
Can verify by enabling the discard protocol on port 9 and running
the following script.

```
from gevent.monkey import patch_all
patch_all()
import gevent

from nylas.logging import configure_logging
from inbox.ignition import engine
from inbox.instrumentation import GreenletTracer
configure_logging()

tr = GreenletTracer()
tr.start()

def func():
    # All data to port 9 gets discarded.
    eng = engine('test', 'mysql+mysqldb://user:pass@127.0.0.1:9')
    eng.execute('SELECT 1').fetchall()

g = gevent.spawn(func)
g.join()
```

Reviewers: spang

Reviewed By: spang

Differential Revision: https://phab.nylas.com/D2538

jobscore/sync-engine,closeio/nylas,nylas/sync-engine,nylas/sync-engine,jobscore/sync-engine,nylas/sync-engine,jobscore/sync-engine,closeio/nylas,closeio/nylas,closeio/nylas,nylas/sync-engine,jobscore/sync-engine