MEDIUM: peers: drop the stick-table lock before entering peer_send_teachmsgs()

MEDIUM: peers: drop the stick-table lock before entering peer_send_teachmsgs()

The function drops the lock very early, and the only operations that
are performed on the entry code are updating the current peer's
last_local_table, which doesn't need to be protected. Thus it's
easier to drop the lock before entering the function and it further
limits its scope.

This has raised the peak RPS from 2050 to 2355k/s with a peers section on
the 80-core machine.