Add DB.Check().

> Work-in-progress

This pull request adds a consistency check to `DB` and `bolt fsck` to the CLI.

It currently implements the following checks:

* Check that all reachable pages are branches or leafs.
* Check that all pages below the high water mark are either reachable or in the freelist.

I still need to add more checks but this uncovered an issue in `TestBucketDeleteQuick` that I still need to resolve.

Fixes: #96

/cc @pkieltyka @tv42