Fix blocking advisory lock in `process_gateways_connections`

[r4victor]

Related to #3551

Currently dstack replicas take blocking advisory lock for processing gateway connections (check_or_restart() and try_collect_stats()):

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

Lines 81 to 92 in ad9fd69

	async def _process_active_connections():
	connections = await gateway_connections_pool.all()
	# Two server processes on a single host cannot process
	# gateway connections and init gateway connections concurrently:
	# Race conditions cause conflicting tunnels being opened.
	async with get_session_ctx() as session:
	async with advisory_lock_ctx(
	bind=session,
	dialect_name=get_db().dialect_name,
	resource="gateway_tunnels",
	):
	await asyncio.gather(*(_process_connection(conn) for conn in connections))

This means replicas can hold db connections open for some time potentially and that should be avoided as much as possible. A solution could be non-blocking pg_try_advisory_lock, but we need to ensure all the replicas can processes eventually.