Make AAE rehash delete missing data from relevant hashtree

This change ensures that AAE does not attempt to continually repair
deleted keys that an AAE hashtree believes still exist.
Change AAE hashtrees to buffer/batch deletes

AAE already buffers writes and submits them as a single write-batch to
LevelDB for better performance. Since deletes are also valid batch
operations, this change buffers deletes alongside writes and submits
all operations as a single write-batch as necessary.

This change also fixes a potential bug in the previous approach that
could reorder a write to happen after a delete, even if the delete
occured after the write -- thus reviving a dead hash.
Merge pull request #478 from basho/jdb-aae-deleted