Disable lazy init of inode table and journal.

Now that discard with guaranteed zeroing is supported by PD,
initializing them is really fast and prevents perf from being affected
when the filesystem is first mounted.
Enable discard when mounting volumes.

This allows the file system to tell the underlying volume that it is not
interested in blocks, for example, when the files are removed.
Merge pull request #68 from wedsonaf/master

* Disable lazy init of inode table and journal when formatting volumes.
* Enable discard when mounting volumes.