Optimize auto evict to minimize coprocessor request latency by avoiding
periodic eviction and loading of regions with high MVCC amplification.

* Do not evict cached regions when memory has not reached the
  `stop-load-threshold`.
* Replace MVCC amplification-based auto evict strategy with a simple
  moving average of coprocessor requests. This is because
  `RegionWriteCfCopDetail` only reflects MVCC amplification in IME,
  not in RocksDB, making low MVCC amplification a poor indicator for eviction.

Optimize auto evict to minimize coprocessor request latency by avoiding periodic eviction and loading of regions with high MVCC amplification.