Fix queue weight behavior so that calls in low-weight queues are not inappropriately blocked.

(This is copied and pasted from the review request I made for this patch)

Asterisk has some odd behavior when queue weights are used. The current logic used when
potentially calling a queue member is:

If the member we are going to call is part of another queue and _that other queue has any 
callers in it_ and has a higher weight than the queue we are calling from, then don't try 
to contact that member. The issue here is what I have marked with underscores. If the 
higher-weighted queue has any callers in it at all, then the queue member will be unreachable 
from the lower-weighted queue. This has the potential to be really really bad if using a 
queue strategy, such as leastrecent or fewestcalls, with the potential to call the same 
member repeatedly.

The fix proposed by garychen on issue 13220 is very simple and, as far as I can see, works 
well for this situation. With this set of changes, the logic used becomes:

If the member we are going to call is part of another queue, the other queue has a higher 
weight than the queue we are calling from, and the higher weight queue has at least as many 
callers as available members, then do not try to contact the queue member. If the higher 
weighted queue has fewer callers than available members, then there is no reason to deny 
the call to this member since the other queue can afford to spare a member.

Since the fix involved writing a generic function for determining the number of available 
members in the queue, I also modified the is_our_turn function to make use of the new 
num_available_members function to determine if it is our turn to try calling a member. There 
is one small behavior change. Before writing this patch, if you had autofill disabled, then 
if you were the head caller in a queue, you would automatically be told that it was your 
turn to try calling a member. This did not take into account whether there were actually any 
queue members available to take the call. Now we actually make sure there is at least one 
member available to take the call if autofill is disabled.

(closes issue #13220)
Reported by: garychen

Review: http://reviewboard.digium.com/r/202/



git-svn-id: http://svn.digium.com/svn/asterisk/branches/1.4@185031 f38db490-d61c-443f-a65b-d21fe96a405b

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