Stop DeleteSpammer from freezing the daemon while pausing a running spammer

[damilolaedwards]

Summary

• Deleting a running spammer called Pause while still holding the spammer map lock for write, and Pause can block for up to 10 seconds waiting for the scenario to notice cancellation. Every other reader, including fetching a single spammer or listing all of them, froze for that entire window since they all need the same lock.
• Released the lock around the Pause call and reacquired it afterward, matching the pattern already used for group deletes. Since the lock is briefly released, a concurrent delete of the same spammer could finish in the meantime, so added a recheck that the entry is still present before continuing.

Test plan

• Added a test confirming a read no longer blocks while a delete is paused inside Pause.
• Added a test confirming the reacquire recheck returns cleanly when a concurrent delete already removed the entry.
• Both verified to fail against the previous behavior and pass with the fix, under -race.