Bug 5169: StoreMap.cc:517 "!s.reading()" assertion (#918)

Bug 5169: StoreMap.cc:517 "!s.reading()" assertion (#918)

unlockSharedAndSwitchToExclusive() was not properly failing when future
readers violated preconditions for obtaining an exclusive lock:

- `if (!readers)` means no old readers
+ `if (!readLevel)` means no old readers and nobody is becoming a reader

That missing "becoming a reader" condition covers a lockShared() caller
that had passed their writeLevel test before we incremented writeLevel
to lock other lockShared() callers out. That caller increments `readers`
while unlockSharedAndSwitchToExclusive() makes `writing` true.

Introduced in commit 1af789e due to poor code duplication. This change
also removes that code duplication by adding finalizeExclusive().