Defragmenting of Blocks used with Tuple Merge

[mjgiardino]

Once the TupleMerge is fixed, a stretch goal should be to allow the defragmentation of the blocks that have had tuples removed but remain in the backing store. This may be able to use the migration mechanisms introduced by @mikelj and/or the separate migration thread developed by @zheguang. When I block is more than half empty, it should be merged with another half empty block, and all EvictedTuples should be updated with the new blockID.