Problem: Defrag was implemented before custom bolt options were added.
Currently defrag doesn't restore backend options.
For example BackendFreelistType will be unset after defrag.
Solution: save bolt db options and use them in defrag.
Thanks to this change:
- all the maps bucket -> buffer are indexed by int's instead of
string. No need to do: byte[] -> string -> hash conversion on each
access.
- buckets are strongly typed in backend/mvcc API.
