Add connection check

Add connection check

cynara_async_check_cache should return CYNARA_API_CACHE_MISS if
disconnected.
cynara_async_create_request and cynara_async_cancel_request should try
reconnecting if disconnected and return CYNARA_API_SERVICE_NOT_AVAILABLE
on failure.
As cynara_async is connecting asynchronously, it could take connection
in progress state which is resolved in cynara_async_process and treated
as connected in other functions. As this state exists after reconnecting,
there should be no cache entries. cynara_async_process should complete
connecting if needed by waiting for write event. If connection fails in
cynara_async_process, then CYNARA_API_SERVICE_NOT_AVAILABLE should be
returned.

Change-Id: I2920f4dbd0bde3e7be74096db33f4ff465bc5028