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.
• handle 481 response in client subscription automatically, so make subscription refresh for that particular buddy occurs more rapidly.
• 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