Fix race in LFU maintenance for add-remove-add of the same key

[bitfaster]

• Evict removes via KVP, instead of just key. This prevents incorrect removal of nodes added to the dictionary when there are two nodes in the write buffer/LRUs with the same key. For example:
  • Node 1 is added with key x
  • Node 1 is removed, now exists only in write buffer + LRU list
  • Node 2 is added with key x, and is also in the write buffer + LRU list
  • Maintenance previously found node 1, and deleted key x from the dictionary detaching node 2. Delete is now via KVP, so node 2 will not be deleted in this situation.
• Check for removed nodes during EvictFromMain()
• Evict() marks items as removed/deleted
• Revise Trim logic to use LRU count instead of the dictionary. If TryRemove has been called, the dictionary can contain fewer items than the LRU lists. First call Maintenance() to flush buffers and mitigate this.
• Enable node detachment integrity check as part of soak tests

[coveralls]

coverage: 99.181% (-0.06%) from 99.237%
when pulling 57d8d97 on users/alexpeck/updatestate
into 9441fa9 on main.