feat: storage block id prefixed with blockid(sha1)

The storage block id is now prefixed with the actual sha1 of the
blockdata. i.e block id

Some of the benefits:

1.We no longer need to query metadata to find the block id for the
corresponding storage id.
2. A performance boost for validation and cleanup service
3. Easy identification of oprhaned blocks if there is more than one
block that begins with the same sha1 in storage.
fix: separator must be '&' or ';'

- pep8 fix
- rename a test correctly
fix: simplify checking of separators
bugfix: os.listdir() is not sorted

explicitly sort, before applying marker or limit logic
Merge remote-tracking branch 'upstream/master' into prefixsha1_storageid

Conflicts:
	deuce/drivers/blockstoragedriver.py
Merge pull request #195 from TheSriram/prefixsha1_storageid

feat: storage block id prefixed with blockid(sha1)