feat: change naming convention between metadata and storage

metadata now stores both blockid and storageid.

The block is now referred in the metadata as blockid (as before)
The block is referred via its storageid in block storage. i.e The block
is stored by the name of storageid

The storage id is generated using a uuid5, which uses a default namespace of
uuid.NAMESPACE_DNS and blockid as the name.
pep8: pep8 fix, and remove unused import
Merge remote-tracking branch 'upstream/master' into meta_storage_name_change
fix: storage id's uuid5 generation using both timestamp and blockid

- Cassandra and Mongo now natively store uuid5, Sqlite doesnt have
  support for it, so its going to be text there.

- Updates all the metadata and storage drivers to make use of the change

- storage_id method is now part of the BlockStorageDriver Abstract Base
  Class
Merge remote-tracking branch 'upstream/master' into meta_storage_name_change
fix: head block

_storage_has_block now checks for a block in storage
using the storageid instead of the block id.
fix: move UUID conversion to metadata drivers

Now, the get_storage_id function generates uuid5s' and returns them
after casting to str.

The individual metadata drivers themselves transform it to UUID format.
feat: allow query of blockid using storage id in metadata

Notes:
- Cassandra: added an index on storageid.
  For eg: It would allow queries to use storage id in the 'where'
  clause while using CQL
fix: add hostname to uuid5 generation
fix: change storage_id to be a static method
fix: make use of static method of BlockStorageDriver in Tests

- Instead of instantiating a Disk Storage Driver, we directly invoke
  the static method 'storage_id'

- rm extraneous parameter 'vaultid'
feat: add x-storage-id header to HEAD block endpoint

The head block endpoint now returns a header as x-storage-id which
contains the name under which the block is stored in block storage

the metadata driver functions have been renamed for additional context
Merge pull request #173 from TheSriram/meta_storage_name_change

feat: change naming convention between metadata and storage