Carryover items from 2.0.33 code review [JIRA: RIAK-2990]

[matthewvon]

• riak_object's KeyGetBucket() and its support routines not reviewed since unused in this branch.

• riak_object_test.cc only returns true for tests of ValueGetLastModTime(). Need some negative tests.

• too many "magic constants" in the code. prefer to have descriptive names.

• declare ExpiryModule::CreateExpiryModule() as a friend, which forces everyone to allocate via your factory method. The only complication is in the unit tests