Add CP hooks in RegionObserver for in memory compaction

Add 4 methods in RegionObserver: preMemStoreCompaction preMemStoreCompactionCompactScannerOpen preMemStoreCompactionCompact postMemStoreCompaction preMemStoreCompaction and postMemStoreCompaction will always be called for all in memory compactions. Under eager mode, preMemStoreCompactionCompactScannerOpen will be called before opening store scanner to allow you changing the max versions and TTL, and preMemStoreCompactionCompact will be called after the creation to let you do wrapping.