Improper locking usage

[r4victor]

There multiple places in dstack codebase related to the usage of the app-level locking mechanism that may cause concurrency issues, weird behavior, and possible freezes like #1241:

• PROCESSING_POOL_LOCK is acquired for potentially a long time to terminate instances:

  dstack/src/dstack/_internal/server/background/tasks/process_instances.py

  Line 699 in a0bc33c

  async with PROCESSING_POOL_LOCK:

• process_instances does not release locks until all instances are processed:

  dstack/src/dstack/_internal/server/background/tasks/process_instances.py

  Line 119 in a0bc33c

  PROCESSING_POOL_IDS.difference_update(i.id for i in instances)