…atch pre 2.1 vclocks which require a rewrite to be safe

Added riak_kv boolean environment variable `hash_only_vclock` which
enables/disabled the vclock hashing logic. Additionally, when enabled,
we will no longer do the expensive compare/logging on read-repair to
catch old pre2.1 vclocks who require a rewrite.

Additional logic must be added to `yz_kv.erl` so the hashes in the
entropy_data element of the Solr documents match what is stored in KV
AAE. Upgrading the hash logic requires clearing all YZ documents and
re-indexing everything. This is because we store the obj hash in the
Solr document. This will hopefully change in the future to allow for
easier AAE changes.

Add a static version “v0” to the data_root of the riak_kv AAE
directory. This allows us to isolate old/newer trees as well as support
downgrades which will not allow old and new trees to exchange either
locally or across MDC.

New error `bad_version` added to the return from
`riak_kv_index_hashtree:do_get_lock` if the requested version and the
local version of the hash tree do not match. Requires changes in
yokozuna and riak_repl to fully function.

Change boolean configuration variable to integer version starting with
0.

Change all specs from atom() to non_neg_integer() for version number.

Add extra logging for invalid non-integer hash version configurations.

Was not correctly handling case where Result list was empty of valid
object responses or only a single valid response.

Add version 0 of object hashing(config `object_hash_version`) which
hashes only the vclock to represent a version of an object. Also add
versioning to index_hashtree get_lock API which now takes a Version
parameter. The absence of a version is assumed to be `undefined` and
will only be allowed to get a lock if the hash on disk is in legacy
format.

Upgraded hash trees are located within the anti_entropy data_dir under
the directory v0. Currently the only supported version is 0 but the
code allows for support of increasing integer versions.

Add verbose riak_object:equals check to get_core for all read
operations who do not trigger read repair. This logic is to detect
potentially pre-2.1 data which may be impacted by old vector clock bugs
and should be rewritten before upgrading the hash version for safety.

WIP commit in case of computer failure or something like that.

…l recovering some history

Also, add some testing logs and debug functions.

Additionally, cleaned up the anode code which killed the hash tree on
upgrade. Removed one binary layer from the riak_object:hash and added
the build locks(different from concurrency lock(doh)) to the upgrade
procedure so we won’t kill trees and then have them sit empty until we
can get a build lock.