AST-2020-002 - res_pjsip: Stop sending INVITEs after challenge limit.

AST-2020-002 - res_pjsip: Stop sending INVITEs after challenge limit.

If Asterisk sends out and INVITE and receives a challenge with a
different nonce value each time, it will continually send out INVITEs,
even if the call is hung up. The endpoint must be configured for
outbound authentication in order for this to occur. A limit has been set
on outbound INVITEs so that, once reached, Asterisk will stop sending
INVITEs and the transaction will terminate.

ASTERISK-29013

Change-Id: I2d001ca745b00ca8aa12030f2240cd72363b46f7