Revamping of presence management to make it more efficient

[bennylp]

Some issues to be fixed in the presence management in PJSUA-LIB:

• pjsua_pres_refresh() will be called every time there is an operation to a buddy, which is not very efficient since this function scans the buddy list for modifications.
• the use of LOCK_BUDDIES/LOCK_BUDDIES (introduced by #351) has been identified to have caused slow performance, especially on mobile devices, since it prevents other thread from working with different buddy object every time the main thread does something with a buddy.
• buddy may need individual timer, so that refreshes do not happen all at once.
• add the last subscription status code (not just reason string) in the buddy info, so that application may handle some other failures with the buddy subscription
• sending of PUBLISH in relation to REGISTER
• handle several cases where PJSUA-LIB should re-subscribe to buddy's presence immediately, rather than relying on the timer:
  • 481 response
  • subscription is terminated with "deactivated" reason

[bennylp]

Added to resubscribe immediately if subscription is terminated with "deactivated" reason. From the RFC:

   deactivated: The subscription has been terminated, but the subscriber
      SHOULD retry immediately with a new subscription.  One primary use
      of such a status code is to allow migration of subscriptions
      between nodes.  The "retry-after" parameter has no semantics for
      "deactivated".

Thanks Saúl Ibarra for the suggestion