threads: add initial thread support

This is a really dumb and broken version of threads, but it will be our
foundation for future changes.

All it currently does is load whatever notes we have locally for a
thread in chronological order. It currently does not open any
subscriptions. It is not clear what is replying to what, but hey, its a
start.

Signed-off-by: William Casarin <jb55@jb55.com>

src/lib.rs

@@ -27,6 +27,7 @@ pub mod relay_pool_manager;
 mod result;
 mod route;
 mod test_data;
+mod thread;
 mod time;
 mod timecache;
 mod timeline;