adding batching options for RPC client

[Chengxuan]

This PR adds an option to leverage JSON-RPC batching feature.

The batch option is disabled by default and the following parameters can be configured to tune its behaviour:

1. batch size, a batch request will be sent when the number of queued requests reaches this number.
2. batch delay, an interval to submitted queued requests as batches no matter how many requests are in the queue
3. batch worker count: the maximum number of concurrent workers (go-routine) for submitting batches

[peterbroadhurst]

Looks great @Chengxuan - please see the feedback on the config/API which I think is mandatory to apply.

The algorithm and config spelling, please have a think about.

I do think the batch timer starting from the first message, rather than being a tick, is worth considering

[Chengxuan]

@peterbroadhurst this PR is ready to be reviewed again. Thanks for the detailed review.

I did start my PR based on the batching logic in txwritter. But then I realized the logic here doesn't need to get as complex as such, a queue-based dynamic dispatch thread allocation with a concurrency control is sufficient. it avoids unnecessary dangling go routine and simplifies the logic for the reader to read in my view.