conn: fix memory leak when using long timeouts
conn: fix memory leak when using long timeouts

A timer will only be garbage collected once it has fired, if a call
returns before the timeout is reached then it will not be garbage
collected, leading to potentially many timers on the heap.

fixes #851

vrischmann/gocql,gocql/gocql,mailgun/gocql,znly/gocql,revinate/gocql,Zariel/gocql,mailgun/gocql,Zariel/gocql,gocql/gocql,znly/gocql,revinate/gocql,vrischmann/gocql