Mv tuning4

[matthewvon]

Discussion here: https://github.com/basho/leveldb/wiki/mv-tuning4

[kellymclaughlin]

• [x] manual verification of code
• [x] eleveldb eunit tests pass
• [x] riak_kv_eleveldb tests pass
• [x] verified the functionality with concurrent riak_core work to change AAE to use prefetch

[kellymclaughlin]

:+1: Nice work!