Re-register automatically on TCP/TLS transport disconnection

[bennylp]

PJSUA-LIB should re-register automatically once it detects that the underlying TCP/TLS transport connection has been disconnected.

Thanks Klaus Darillion and Fabio Pietrosanti for the request.

Specifications:

1. Add new field in pjsua_acc_config, i.e: reg_retry_interval to configure the interval of auto re-registration when registration failed of timeout or transport disconnection.
2. Add new field in pjsua_acc, i.e: route_down, a flag to specify whether the transport used for contacting the first route of the account is down.
   • If this flag is set, sending SIP messaging via PJSUA-LIB API (except the auto re-register) will be blocked (by returning an error code) immediately.
   • This flag will be set/reset automatically by PJSUA-LIB based on events such as transport disconnected, registration timeout, registration succeeded.
   • This flag will only be set when account has route-set (global proxy or account specific proxy is configured) and registrar is set.

Notes:

• In the case of IP address changed, it should be better for application to restart pjsua-lib (e.g: to update media transport address binding).