Remove ensemble backend syncing logic

riak_ensemble now supports an integrated integrity checking mechanism,
and therefore no longer requires nor supports backend based trust and
syncing logic. This commit removes all such logic from riak_kv.
Update consistent AAE to simply use read repair

The previous approach for consistent AAE was designed to function as the
backend syncing mechanism for K/V ensembles. Now that riak_ensemble no
longer requires backend syncing, this approach is no longer necessary
nor is it correct.

This commit completely removes the previous approach, reducing consistent
AAE to simple read repair now that riak_ensemble has a read repair mechanism.
This not only simplifies things, but this approach also properly works with
the new integrated integrity/validation logic in riak_ensemble.
Add synctree_path callback to ensemble backend

This commit adds the new synctree_path callback to the K/V ensemble
backend. This callback is used to override how riak_ensemble maps
peer synctrees to on-disk LevelDB instances.

This commit implements the callback using a M:N mapping, where all
peers that correspond to a single vnode map to a common LevelDB
instance. This makes the number of synctree LevelDB instances scale
with the size of the ring, rather than the number of distinct preflists.

This is the same approach used for AAE in hashtree.erl.

Requiring fewer instances makes better use of file handles, write buffers,
and internal LevelDB caches.
Remove final remnants of ensemble backend syncing

Update riak_kv_ensemble_console to no longer talk about trust or AAE,
as backend based trust and AAE syncing has been completely removed.

The console output now references concepts relevant to the new
synctree approach directly integrated into riak_ensemble.

Remove the trust setting from riak_kv.schema since the setting no
longer exists. Providing schema mappings for the new relevant settings
is left for a future commit.
Update spec for riak_kv_ensemble_console:ping_quorum/1
Merge pull request #1002 from basho/jdb-ensemble-integrity

Update to latest riak_ensemble integrity approach

Reviewed-by: andrewjstone