Memstore's heap size will be decreased to minus zero after flush

When creating an active mutable segment (MutableSegment) in memstore, MutableSegment's deep overheap (208 bytes) was added to its heap size, but not to the region's memstore's heap size. And so was the immutable segment(CSLMImmutableSegment) which the mutable segment turned into (additional 8 bytes ) later. So after one flush, the memstore's heapsize will be decreased to -216 bytes, The minus number will accumulate after every flush.
CompactingMemstore has this problem too.

We need to record the overhead for CSLMImmutableSegment and MutableSegment to the corresponding region's memstore size.

For CellArrayImmutableSegment, CellChunkImmutableSegment and CompositeImmutableSegment , it is not necessary to do so, because inside CompactingMemstore, the overheads are already be taken care of when transfer a CSLMImmutableSegment into them.