When creating a CryptStream, the last blob is always empty. Previously, this
blob wouldn't be deleted and would instead just stick around in the blobfiles
directory.
* Move the blob verification to the actual Blob object
* remove the check on verification time
* remove get_blob_length from BlobManager
Removed because I'm not sure what checking verification time against ctime gets us, except some protection against an accidental modification of the blob.
In the next commit I change conf.settings to be initialized
at runtime instead of load time and so any import of
`from lbrynet.conf import settings` will be and stay None if
it happens before the initialization.
