Revert "Rewrite the JOIN channel logic, dropping MaxJoins"

This reverts commit db7c47f97d4b18bce7264af5db6331fc296ff69a.

Too many joined channels at once started to cause disconnect because of
"Max SendQ Exceeded", which is not much better than previous Flood.

Now MaxJoins is 0 by default, which preserves the current behavior of
joining all channels at once. If someone experiences those disconnects
due to SendQ, they can tune MaxJoins.

Fix #329

Conflicts:
	include/znc/User.h
	modules/controlpanel.cpp
	modules/webadmin.cpp
	src/User.cpp