Keep-alive mechanism for TCP and TLS transports

[bennylp]

TCP and TLS transports should periodically transmit packet to keep the connection and NAT binding open.

[bennylp]

Done in r1473:

• Implemented keep-alive mechanism for TCP and TLS transports. The keep-alive mechanism is controlled by two settings in pjsip/sip_config.h: PJSIP_TCP/TLS_KEEP_ALIVE_INTERVAL, to control the interval, when this value is zero keep-alive mechanism will not be used, and PJSIP_TCP/TLS_KEEP_ALIVE_DATA to specify the payload to be sent with the packet.
• TCP transport idle-time-before-closing value (PJSIP_TRANSPORT_IDLE_TIME setting) is increased from 60 seconds to 600 seconds.
• Because of ticket #390, TLS transport is no longer kept indefinitely in the stack. Previously the reference counter for TLS is initialized to 1, which means that TLS transport will be kept indefinitely in the stack until it is explicitly shutdown by application.

[bennylp]

Fixed in r1476:

• keep-alive timer is not canceled when TLS transport is destroyed, causing crash in the timer heap.