All use of sortedcache will need this header, so put it in the
definition of the sortedcache API.

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.

This adds a convenient new data type for caching the contents of
file in memory when each item in that file corresponds to a name
and you need to both be able to lookup items by name and iterate
over them in some sorted order.  The new data type has locks in
place to manage usage in a threaded environment.