LRU enumeration should not return expired/removed items

[bitfaster]

This fixes a small gap, and introduces an observability difference because TLRU tests previously used to the count to observe eviction. To fix this:

• Remove the abstract base class used for TLRU tests - this is no longer needed since introducing Duration and having a single implementation of TLruLongTicksPolicy.
• Change the tests to use ConcurrentTLru, and verify expiry via the internal queue counters which are part of the public API.

[coveralls]

coverage: 99.106% (-0.2%) from 99.28%
when pulling 37ad62c on users/alexpeck/filterenum
into 6339ed9 on main.