Synchronous deletion of doc level queries during delete monitor operation

[eirsep]

During delete monitor operation, after monitor is deleted, its doc level queries are deleted. But currently doc level queries are being deleted in async fashion.

This should be done in synchronous manner as custom query indices are allowed and alerting plugin stakeholders may want better manageability of the custom query index.

For ex. they might wanna delete the custom query index if it becomes empty as a result of monitor deletion. This can only be achieved if delete monitor operation waits for doc level queries deletion and then responds.