* fix: return read buffer size in libutp read buffer size callback
* refactor: clamp amount of data processed in `can_read_wrapper()`
* chore: housekeeping
* refactor: call `utp_read_drained()` in on-read callback
so that uTP acks can be sent out in a more timely fashion
(cherry picked from commit 5000edef01)
Its two calls to evbuffer_peek() used 3.5% of CPU use (measured with perf
when built with RelWithDebInfo). I added vecs() so that libtransmsision
could send noncontiguous buffers via utp_writev(); but in my testing, all
the buffers being sent are contiguous and so this is unnecessary work.
* refactor: make tr_peer_socket.type private
* refactor: reimplement tr_peerIo::address() as a wrapper around tr_peer_socket::address()
* refactor: remove tr_address, tr_port from tr_peerIo
* refactor: replace tr_netClosePeerSocket() with tr_peer_socket::close()
