Fix race between creating collective groups and submitting host tasks

[fknorr]

Current Celerity has a race condition on worker nodes between submission of collective host tasks and the submission of their kernels to the host queue. Choosing a non-default collective group creates an additional worker thread from the main thread, and kernel submission requires that thread to be up and running.

• The executor busy-waits for the task definition to be available, but the collective group thread was created after task insertion into the ringbuffer. This has been reversed to ensure the worker thread is ready when the task becomes available to the executor.
• Even though the set of worker threads in host_queue was accessed by both main and executor thread, it was left unsynchronized. The class member is now protected by a mutex.

[psalz]

LGTM!