New pjsua_destroy2() API to allow shutting down the library without sending any outgoing messages

[bennylp]

New API is proposed by this ticket:

PJ_DECL(void) pjsua_destroy2(unsigned flags);

The flags argument may contain bitmask of the following options:

• PJSUA_DESTROY_NO_RX_MSG: allow sending outgoing messages (such as unregistration, event unpublication, BYEs, unsubscription, etc.), but do not wait for responses. This is useful to perform "best effort" clean up without delaying the shutdown process waiting for responses.
• PJSUA_DESTROY_NO_TX_MSG: do not send any outgoing messages at all. This flag is useful if application knows that the network which the messages are to be sent on is currently down. This option implies PJSUA_DESTROY_NO_RX_MSG flag.