Is your feature request related to a problem? Please describe.
The azure.servicebus.AutoLockRenewer is implicitly limited to renewing locks for at most max_workers number of messages. For an application that takes many messages off of the queue for parallel processing, this can present problems.

The azure.servicebus.AutoLockRenewer uses a single future inside of a ThreadPoolExecutor to monitor messages and renew locks. This means it's limited to monitoring at most max_workers messages at a time (since a thread pool only runs one future per-thread until the future completes).

Describe the solution you'd like
An alternative implementation could instead use max_workers futures monitoring a set of messages per future rather than a single message per future.

Describe alternatives you've considered
An alternative solution would be to implement parallelism with each worker taking a message off of the queue for itself to process. This is certainly possible, but there are still cases where taking more than max_workers messages off of the queue could be desirable (e.g. prefetching messages).