Task lock should be non-local

[thomasst]

Analyzing the thread safety of the heartbeat function in the sync executor:

• Redis-py should by threadsafe by default unless single_connection_client is given, which is false by default.
• When doing a heartbeat() we update the task timestamp, renew any task locks and any queue lock.
• The queue lock (Semaphore) does not care about threads so we can therefore renew it from a different thread.
• Redis locks, which we use for locking tasks, are thread-local by default, we therefore need to set thread_local=False.
• We use Redis locks in a couple other places but these are not being accessed by the heartbeat thread.

Also:

• Improved heartbeat so we don't need to do a heartbeat if the task already completed after the wait, and added exception handling.
• Fixed test to give it a bit more time to start (had some failures locally) + have at least one task & queue lock that should be renewed.