Fix LRU clear when warm/hot items are touched

[bitfaster]

When items are touched, they are moved to warm by cycling. The Clear() logic naively attempted to cycle the queues n times, where n is the number of items in the cache. Any items that were touched would be cycled to warm, and would fail to be removed.

Fix summary:

• Consolidate Clear() logic to call into TrimLiveItems(), which tracks how many items have been removed.
• All hot and warm items can cycle into warm. Therefore, to guarantee that we can remove itemCount items, we must cycle capacity.Warm + capacity.Hot times. Fixed TrimLiveItems() to take this into account.
• Implemented unit tests for Trim() and Clear() that verify touched items are removed correctly.

[coveralls]

coverage: 98.493% (-0.07%) from 98.56%
when pulling 6f2dfb0 on users/alexpeck/lruclear
into dbb6d7b on main.