Fixed Buddy to use correct account during subscription

[sauwming]

To fix #4226

The issue occurs because in pjsua, buddy is not tied to any account, so it will always call pjsua_acc_find_for_outgoing() to find the appropriate account for outgoing subscription.

Also in this PR:
Fixed bug in Account::findBuddy2() and enumBuddies2() to only return buddies associated with the account.

[nanangizz]

What happens to the buddy when the account is deleted?

[sauwming]

What happens to the buddy when the account is deleted?

Good question. I just add validity check in pjsua. In pjsua2 though, if the associated Account instance has been destroyed, then it will be a problem indeed, though I suppose it will be the app developer's fault? The best we can do is add doc in Buddy::create().

[nanangizz]

• The validity check may not work properly (returning false validity) when the original & deleted account ID is being reused.
• Alongside docs, perhaps what if account deletion also raises assertion (and return an error) for PJSUA & exception for PJSUA2 if the account still has buddies referring to it? For app using PJSUA2 this might be a surprise. For app using PJSUA this should not be a problem as linking buddy to account is new in this PR.

[sauwming]

I tried returning error but encountered a few issues:

• Account::shutdown() doesn't throw error.
• Most of the time, people don't expect deletion to fail. So they don't check return status of pjsua_acc_del() and also there's no retry mechanism, so they won't expect that the account is still there until they run out of capacity.

So, I opted to remove the buddy-account association. There's still a possibility of race, but then the doc has specifically warned (in three locations now) to keep the Account alive when Buddy is alive:

pjproject/pjsip/include/pjsua2/presence.hpp

Lines 206 to 208 in cb85368

	* create()) will unregister and delete the buddy from PJSUA-LIB. Application
	* must delete all Buddy instances belong to an account before shutting down
	* the account (via Account::shutdown()).

(the other two warnings in pjsua_buddy_config and Buddy::Create() in this patch).

Also fixed bug in Account::findBuddy2() and enumBuddies2() to only return buddies associated with the account.