In-memory-Engine: memory usage metrics and overhead reduction

[tonyxuqqi]

Currently the in-memory-engine's memory consumption is much more than block cache.
In this case, when in-memory-engine is disabled, the block cache size is 8.4GB (That's all data cached). But when the in-memory-engine is enabled, the memory usage by in-memory-engine is more than 30GB, about 4x of block cache size.

The possible reason is that eviction is not implemented and there're some follower region's data in the in-memory-cache. But we need a clear metrics to show the memory usage, cached key count, cached region count, etc.