Limit number of items in memory cache

[JunTaoLuo]

We need to add an optional count limit on the memory cache to ensure a there is a maximum limit on the memory use of MemoryCache. When the count limit is reached, we will trim a percentage of the items in the cache (we have used 10% in the past).

cc @muratg @Eilon

Post 2.0 we plan to include a size hint to the memory cache and design more extensible eviction policies.

[JunTaoLuo]

Followup item filed in #326

[JunTaoLuo]

We decided to implement #326 instead.