MEDIUM: peers: drop then re-acquire the wrlock in peer_send_teachmsgs()

MEDIUM: peers: drop then re-acquire the wrlock in peer_send_teachmsgs()

This function maintains the write lock for a while. In practice it does
not need to hold it that long, and some parts could be performed under a
read lock. This patch first drops then re-acquires the write lock at the
function's entry. The purpose is simply to break the end-to-end atomicity
to prove that it has no impact in case something needs to be bisected
later. In fact the write lock is already dropped while calling
peer_send_updatemsg().