Simplify synchronization semantics of `PrimaryOnlyService`

The main intention here is to simplify the waiter/notifier pattern around PrimaryOnlyService::_rebuildInstances (defined here). In particular, callers to PrimaryOnlyService::getOrCreateInstance, PrimaryOnlyService::lookupInstance, and PrimaryOnlyService::getAllInstances block on _rebuildCV using the following:

opCtx->waitForConditionOrInterrupt(_rebuildCV, lk, [this]() { return _state != State::kRebuilding; });

However, PrimaryOnlyService::_rebuildInstances may call notify_all on this condition variable (i.e., _rebuildCV) even if there's a change in term (example):

...

stdx::lock_guard lk(_mutex);

if (_state != State::kRebuilding || _term != term) {

    _rebuildCV.notify_all();

    return;

}

...

We should simplify/clarify this code and the logic around notifying threads that await completion of PrimaryOnlyService::_rebuildInstances.

Acceptance criteria: clarify when a thread would block on _rebuildCV, what are the events that would stop this wait, and what's the expected behavior for each observed event. Then, modify the code to align with the findings.