This PR is for enabel APC feature over block manager V2.

APC core idea is like maintain a table mapping between content hash and
physical block. If one block is full, then it would be prompted to be
immutable. While if several immutable comes in sequence, we kept the
first physical block, while free the other.

Those freed physical block, aka no reference at scheduler, would be
housekept inside evictor, when we try to alloc block again under pressue,
and find no free one as in the initial state, we could resort to
evictor to pop one unrefered computed block, which is used to store
new data comes from upper layer.

The evictor policy could be changed, but current only implement LRU one.

Co-authored-by: Sage Moore <sagemoore@utexas.edu>