RFC/Pull Request: Refs db backend

I've revived and modified Ronnie Sahlberg's work on the refs db backend.

The work is on top of be3c13e5564, Junio's "First batch for 2.5 cycle". I recognize that there have been changes to the refs code since then, and that there are some further changes in-flight from e.g. Michael Haggerty. If there is interest in this, I can rebase once Michael's changes land.

The changes can be found here: https://github.com/dturner-tw/git.git on the dturner/pluggable-backends branch

The db backend code was added in the penultimate commit; the rest is just code rearrangement and minor changes to make alternate backends possible. There ended up being a fair amount of this rearrangement, but the end result is that almost the entire git test suite runs under the db backend without error (see below for details).

The db backend runs git for-each-ref about 30% faster than the files backend with fully-packed refs on a repo with ~120k refs. It's also about 4x faster than using fully-unpacked refs. In addition, and perhaps more importantly, it avoids case-conflict issues on OS X.

I chose to use LMDB for the database. LMDB has a few features that make it suitable for usage in git:

1. It is relatively lightweight; it requires only one header file, and
the library itself is under 300k (as opposed to 700k for
e.g. sqlite).

2. It is well-tested: it's been used in OpenLDAP for years.

3. It's very fast.  LMDB's benchmarks show that it is among
the fastest key-value stores.

4. It has a relatively simple concurrency story; readers don't
block writers and writers don't block readers.

Ronnie Sahlberg's original version of this patchset used tdb. The advantage of tdb is that it's smaller (~125k). The disadvantages are that tdb is hard to build on OS X. It's also not in homebrew. So lmdb seemed simpler.

To test this backend's correctness, I hacked test-lib.sh and test-lib-functions.sh to run all tests under the refs backend. Dozens of tests use manual ref/reflog reading/writing, or create submodules without passing --refs-backend-type to git init. If those tests are changed to use the update-ref machinery or test-refs-be-db (or, in the case of packed-refs, corrupt refs, and dumb fetch tests, are skipped), the only remaining failing tests are the git-new-workdir tests and the gitweb tests.

Please let me know how it would be best to proceed.