[reSIProcate] DialogUsageManager as registrar

[palladin <p-aladin@xxxxxxxxx>]

Dear resip devels,
resiprocate 1.9.2
could you please advise with the next issue:

- I've installed my own ServerAuthManager into the DUM;
- loaded dum with some REGISTER amount
- did shutdown during this process
-My own ServerAuthManager implementation has number of asynchronous 
workers, which communicate with radius and post auth results using 
UserAuthInfo, application crashed at this call.

As I understand this happens because dum does not check if 
mIncomingFeatureChainMap is not empty on shutdown.

Appreciate in advance.
Sincerely tez.