Skip invalid state manifests in GC bucket and add warning log

See https://github.com/basho/riak_cs/issues/827

Logged information includes:

- Key in GC bucket
- UUID (in manifest)
- CS bucket and key (also in manifest)

It's noteworthy that if riak_cs_delete_fsm replies ok with the same
total blocks and deleted blocks to riak_cs_gc_worker, then the worker
attempt to delete manifest entry in twop_set.  Doing it before
deleting manfiests and blocks completely poses orphan
manifests/blocks.  In this PR, riak_cs_delete_fsm replies with total
blocks = 1 and deleted_blocks = 0.
Add GC riak_test for invalid state manifet in GC bucket
Select PB socket to manipulate GC bucket in the same way as for manifests

Because GC buckets are divided among bags and distribution is
determined by manifests. Manifests at an certain bag goes to
riak-cs-gc bucket in the Riak cluster by delete.
Mimic more precisely
Fix eqc failure caused by riak_cs_delete_fsm:start_link's arity change
Copy twop_set beam file for riak_test
Refine warning log and code comment
Merge pull request #964 from basho/bugfix/skip-invalid-state-manifest-in-gc

Bugfix: Skip invalid state manifests in GC bucket

Reviewed-by: kuenishi