Enhancements to js_PublishAsync

[pramodsri]

/** \brief Publishes data to JetStream but does not wait for a #jsPubAck.
 *
 * See #js_PublishMsgAsync for details.
 *
 * @param js the pointer to the #jsCtx object.
 * @param subj the subject the data is sent to.
 * @param data the data to be sent, can be `NULL`.
 * @param dataLen the length of the data to be sent.
 * @param opts the publish options, possibly `NULL`.
 */
NATS_EXTERN natsStatus
js_PublishAsync(jsCtx *js, const char *subj, const void *data, int dataLen,
                jsPubOptions *opts);

The current signature of js_PublishAsync does not provide a mechanism for a positive ack and does not call the jsOpts.PublishAsync.ErrHandler in the following scenario

>> natsConnection_Connect
>> natsConnection_JetStream
>> sleep (shutdown the server from another shell)
>> js_PublishAsync return NATS_OK
>> jsOpts.PublishAsync.ErrHandler does NOT get called

This issue will track the changes needed to support the following requests:

1. Call jsOpts.PublishAsync.ErrHandler on timeout, the timeout can be set through jsPubOptions.MaxWait
2. Provide parity with Go PublishAsync API by supporting positive ack (along the lines of stanConnection_PublishAsync)

/* 
 * 's' can be NATS_OK, NATS_TIMEOUT or any other error due to async publish.
 * 'msg' handle, it is apps responsibility to free the msg
 */
typedef void (*jsPubAckHandler)(natsStatus s, natsMsg *msg, void *closure);
 
/* 
 * jsPubOptions.MaxWait can be used to control the timeout for the ack handler
 * 'ah' is called on success, async publish error or timeout
 */
NATS_EXTERN natsStatus
js_PublishAsyncAck(jsCtx *js, const char *subj, const void *data, int dataLen,
                jsPubOptions *opts, jsPubAckHandler ah, void *ahClosure);

Thanks for the help !!