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`).