This is a big refactoring of the attribute file cache to be a bit
simpler which in turn makes it easier to enforce a lock around any
updates to the cache so that it can be used in a threaded env.
Tons of changes to the attributes and ignores code.

This adds a basic test of doing simultaneous diffs on multiple
threads and adds basic locking for the attr file cache because
that was the immediate problem that arose from these tests.

This updates the git_pqueue to simply be a set of specialized
init/insert/pop functions on a git_vector.

To preserve the pqueue feature of having a fixed size heap, I
converted the "sorted" field in git_vectors to a more general
"flags" field so that pqueue could mix in it's own flag.  This
had a bunch of ramifications because a number of places were
directly looking at the vector "sorted" field - I added a couple
new git_vector helpers (is_sorted, set_sorted) so the specific
representation of this information could be abstracted.

This converts an internal lock from a write lock to a read lock
where write isn't needed, and also clarifies some doc things about
where various locks are acquired and how various APIs are intended
to be used.

This is the first use we have of pthread_rwlock_t in libgit2.
Hopefully it won't cause any serious portability problems.

This adds two other APIs that I need to the sortedcache type.