Mv async close

[matthewvon]

This is async close code originally released in 1.4.2-turner.

Discussion is here: https://github.com/basho/leveldb/wiki/mv-async-close

[kellymclaughlin]

I rebased this branch from the HEAD of master in order to run the eleveldb tests and it rebases cleanly and all the eunit tests pass.

[kellymclaughlin]

I ran the leveldb unit tests with make check and all tests pass.

[kellymclaughlin]

The 1.4.2-turner branch contained the same changes that are represented here. I used that branch and ran the entire Riak CS riak_test suite. That gives me assurance that this change does not break any critical functionality at least from the perspective of Riak CS.

[kellymclaughlin]

This change has been deployed on a customer's system that was experiencing this issue. We have observed that this change works as intended to resolve the issue.

[kellymclaughlin]

👍 Great work!