PJSUA initialization and shutdown

As described in the ​PJSUA-API Basic API documentation, application needs to call pjsua_create(), pjsua_pool_create(), pjsua_init() to perform the initialization. Then app must call pjsua_start() to start PJSUA and finally after everything is done, call pjsua_destroy() to shut it down.

It is not necessary to call these functions from the main thread, i.e. application can perform the initialization and shutdown from a secondary/worker thread. However, it is recommended to call pj_init() and pj_shutdown() from the main thread.