Use group lock instead of mutex for SIP dialog which is useful for B2BUA scenarios

[riza]

On B2BUA scenario (with 2 leg/dialog):

1. B2BUA receive a BYE on one leg, while the other leg also receive a BYE.
2. While holding the lock on one leg, the B2BUA will try to forward the BYE it received to the other leg. The same goes for the other leg, however the process will stuck since it's unable to obtain the other's dialog lock.

It will lead to a deadlock scenario.

For this scenario, it is useful for both dialog to use the same lock (e.g: group lock).

The patch will introduce new API which will help for the above scenario.

• pjsip_dlg_create_uac2() and
• pjsip_dlg_get_lock()

Thanks to Peter Koletzki for the report.

[riza]

In 6037:

Close #2211: Use group lock for SIP dialog.

[nanang]

In 6042:

Re #2211: Fixed compile error in pjsip_dlg_create_uac() on MSVC2005, var declaration should be at the beginning of function.