Add MxAccessWriteCompletionCache.Remove and RemoveServer and call them
from AlarmAckWriter when it drops an item (dirty re-add, LRU eviction,
failed AddItem/Advise/Write) and when it tears down its registration, so
the cache no longer keeps one entry per dropped handle for the life of
the worker.
Fixes#165
Claude-Session: https://claude.ai/code/session_016J6isfcgqy4SB2RK3ZvFvT