BUG/MEDIUM: connections: Hold the lock when wanting to kill a connection.

BUG/MEDIUM: connections: Hold the lock when wanting to kill a connection.

In connect_server(), when we decide we want to kill the connection of
another thread because there are too many idle connections, hold the
toremove_lock of the corresponding thread, othervise, there's a small race
condition where we could try to add the connection to the toremove_connections
list while it has already been free'd.

This should be backported to 2.0 and 2.1.