Add checking just before creating bucket, ensuring no mp upload is alive

If bucket is not empty and includes unaborted multipart upload, then
stanchion returns 409 with message, expecting Riak CS retries after
deleting all unaborted multipart upload.

This commit prohibits creating bucket if unaborted multipart upload
exists. Former version than 1.4.x has this bug and if there already
exists such buckets, Riak CS is supposed to include a cleanup script
to fix such buckets. Note that that script won't do any mutual
exclusion, thus after upgrading from 1.4.x to 1.5.0, operators should
run that script to enable re-creating those buckets.

Another few trivial fixes:
- future work is to remove list_keys runs twice in checking
- get rid of if clause in stanchion_utils:bucket_availabe/4
- and fixed some logic bucs
- dialyzer is our friend
Merge pull request #78 from basho/bugfix/cs475-check-mpuploads-squashed

Add checking just before creating bucket, ensuring no mp upload is alive