MINOR: connection: pass the thread number to conn_delete_from_tree()

MINOR: connection: pass the thread number to conn_delete_from_tree()

We'll soon need to choose the server's root based on the connection's
flags, and for this we'll need the thread it's attached to, which is
not always the current one. This patch simply passes the thread number
from all callers. They know it because they just set the idle_conns
lock on it prior to calling the function.