mdserver_memory: check the lock holder and block lock calls
mdserver_memory: check the lock holder and block lock calls

In order to test locking, we need to track which `MDServerMemory`
instance took the lock, and block other instances from taking the lock
until the holder releases it (or it expires).

Issue: KBFS-2484

keybase/kbfs,keybase/client,keybase/client,keybase/client,keybase/client,keybase/client,keybase/client,keybase/client,keybase/client,keybase/client,keybase/client,keybase/kbfs,keybase/kbfs,keybase/client,keybase/kbfs,keybase/client