[peer_handler] Take the peers lock before getting messages to send

[peer_handler] Take the peers lock before getting messages to send

Previously, if a user simultaneously called
`PeerHandler::process_events()` from two threads, we'd race, which
ended up sending messages out-of-order in the real world.
Specifically, we first called `get_and_clear_pending_msg_events`,
then take the `peers` lock and push the messages we got into the
sending queue. Two threads may both get some set of messages to
send, but then race each other into the `peers` lock and send the
messages in random order.

Because we already hold the `peers` lock when calling most message
handler functions, we can simply take the lock before calling
`get_and_clear_pending_msg_events`, solving the race.