When Watcher is stopped and there are still outstanding watches running
Watcher will report it self as stopped. In normal cases, this is not problematic.
However, for integration tests Watcher is started and stopped between
each test to help ensure a clean slate for each test. The tests are blocking
only on the stopped state and make an implicit assumption that all watches are
finished if the Watcher is stopped. This is an incorrect assumption since
Stopped really means, "I will not accept any more watches". This can lead to
un-predictable behavior in the tests such as message : "Watch is already queued
in thread pool" and state: "not_executed_already_queued".
This can also change the .watcher-history if watches linger between tests.
This commit changes the semantics of a manual stopping watcher to now mean:
"I will not accept any more watches AND all running watches are complete".
There is now an intermediary step "Stopping" and callback to allow transition
to a "Stopped" state when all Watches have completed.
Additionally since this impacts how long the tests will block waiting for a
"Stopped" state, the timeout has been increased.
Related: #42409
1dab73929f