Add some error handling and retries when creating a new connection

[aigarius]

If connection to DB fails before we hand it off to the application, retry connecting to the DB a few times before failing

[meteozond]

+1 Looks reasonable, are there any drawbacks for this PR?

[GitFree]

if MYSQLPOOL_RETRY = 5, it actually retried 6 times.

[variable]

I think here should raise exception instead of "trying again"

[variable]

I think it should be refactored into:

while(retry and not connection):
    try:
        connection = get_pool().connect(**kwargs)
    except:
        connection = None
        retry -= 1
        if not retry:
            raise
return connection