Fix race in LRU concurrent update and TryRemove(kvp)

[bitfaster]

There is a lock in TryUpdate to prevent concurrent updates, but no item lock on TryRemove(kvp). No lock is needed for TryRemove(key) because updates to the value do not affect the outcome - key should always be removed. The value is important in the kvp variant however (#371), where the equivalent lock was missed leading to a race when the value changes between finding the existing LruItem and removing it.

[coveralls]

coverage: 99.209%. remained the same
when pulling 0fbf72e on users/alexpeck/updremkvp
into f9563f8 on main.