Do not lock terminating jobs in `process_runs`

[r4victor]

Related to #3551

process_runs currently locks terminating jobs only to override termination reason in case of abort:

dstack/src/dstack/_internal/server/services/runs/__init__.py

Lines 991 to 997 in ad9fd69

	unfinished_jobs_count += 1
	if job_model.status == JobStatus.TERMINATING:
	if job_termination_reason == JobTerminationReason.ABORTED_BY_USER:
	# Override termination reason so that
	# abort actions such as volume force detach are triggered
	job_model.termination_reason = job_termination_reason
	continue

In principle, this can be moved to process_terminating_jobs, so that process_runs does not need to lock terminating jobs and then process_runs and process_terminating_jobs can work simultaneously, reducing job lock contention.