Append random number to epoch binary

Every CS DELETE execute GET/PUT to a single BKey = {riak-cs-gc, Epoch}.
Appending random number to Epoch (hopefully) solve this hot key problem.

GC daemon can collect garbage with suffixed key because it accesses
keys by binary range 2i.
Throw away in-line if expression
Make GC key suffix max value configurable by application env

The default value is 256. In terms of scalability, larger value is better.
On the other hand, when there are more keys in GC bucket, listing keys takes
longer. The default value 256 is conservative, safe side.

If GC mechanism will be improved (streaming/multi-threading), we may safely
adopt larger value, for example 1024 or more.
Refactor: some specs and more congruent return types
Merge pull request #689 from basho/feature/gc-key-scattering

Append random number to epoch binary