Add DVID to projects using Bolt

DVID added Bolt as an optional storage engine.
Merge pull request #107 from DocSavage/master

Add DVID to projects using Bolt
Remove count and overflow columns for free pages on 'bolt pages'.

The count and overflow columns are meaningless for freed pages since
there could be random overflow data in there. This commit removes those
columns for free pages.
Merge branch 'master' of https://github.com/boltdb/bolt