RFC/Pull request v2: Refs db backend

RFC/Pull request v2: Refs db backend

[David Turner]

I've prepared another version of my pluggable backends series.

This version has the following changes:
1. Addressed all cosmetic review comments from previous version
2. Significant cleanup of LMDB code: formatting and variable names,
functions extracted, etc.
3. Explicit handling of per-worktree refs and pseudorefs; several new
patches in the series now address this.
4. refdb init function added to backend
5. rename of refs-be to refs-lmdb
6. Replace void * with struct ref_transaction
7. removed a memory leak
8. More documentation
9. Rebased on top of more-recent upstream (and on top of my preamble
patch series).

There are probably a few further changes that I've forgotten.  Because
the patch series is 49 patches, I've made it available for pull:

https://github.com/dturner-tw/git.git on the 
dturner/pluggable-backends branch

Let me know if you think it's ready for per-patch review, and I'll send
all 49 patches to the list.

Thanks.