Revert "Rewrite the JOIN channel logic, dropping MaxJoins"

This reverts commit db7c47f97d.

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

modules/data/webadmin/tmpl/add_edit_user.tmpl

@@ -261,6 +261,11 @@
 					<input type="number" name="jointries" value="<? VAR JoinTries ?>" class="third" min="0"
 						   title="This defines how often ZNC tries to join, if the first join failed, e.g. due to channel mode +i/+k or if you're banned." />
 				</div>
+				<div class="subsection">
+					<div class="inputlabel">Max Joins:</div>
+					<input type="number" name="maxjoins" value="<? VAR MaxJoins ?>" class="third" min="0"
+					       title="How many channels are joined in one JOIN command. 0 is unlimited (default). Set to small positive value if you get disconnected with `Max SendQ Exceeded'"/>
+				</div>
 				<div class="subsection">
 					<div class="inputlabel">Max IRC Networks Number:</div>
 					<input type="number" name="maxnetworks" value="<? VAR MaxNetworks ?>" class="third" min="0"