Add support for new riak_kv access methods get_object/4 and put_object/4

On a 2 node test, 1 box load gen and 1 box a single Riak node with
all defaults (i.e. n_val=3 and ring size=64), when comparing to the
memory backend with 50% get/50% put op mix:

* 1 byte value size: this backend is slightly slower than the memory
backend, by a few percentage points
* 20KB value size: this backend is 17% faster (5834.42 -> 6837.8)
with a drop of 99.9% latency of 50msec -> 40msec