Change AAE to use incremental crypto:sha calculations

The crypto library is implemented as a NIF, and long running NIFs can
negatively impact the work balancing algorithm of the Erlang
scheduler. Using normal crypto:sha(object) is unbounded in duration as
the size of an object is unbounded. This commit therefore changes AAE
to use an incremental SHA calculation (sha_init/sha_update/sha_final)
with a set chunk size (defaults to 4096; configurable app var
riak_kv/anti_entropy_sha_chunk).
Merge pull request #514 from basho/jdb-aae-incremental-sha-1.3