Added HTTP notify support to improve integration with DATUM Gateway

[pdath]

When using the DATUM gateway, you need to use the blocknotify command in Bitcoin Knots. An example is:

blocknotify=wget -q -O /dev/null http://localhost:7152/NOTIFY

This works by shelling out and running wget. I have been profiling the performance of this, and on my slow hardware (Odroid M2 16GB), this takes about 75ms.
This is a problem because from the moment that Bitcoin Knots has accepted a new block, any energy spent by a hasher is wasted until DATUM can get the notification of the new block, issue a getblocktemplate() RPC-JSON call to Bitcoin Knots, get the result, and send a new work unit to the hasher to start working on a new job.

I propose adding a new config option to improve the DATUM gateway integration notification performance, and model it after blocknotify.
httpnotify=http://localhost:7152/NOTIFY/%s

Although DATUM does not use the block hash, I propose keeping the %s option (to match blocknotify). I do personally use the %s blockhash in other applications. It offers many benefits, such as providing a unique reference in logs across different systems when debugging issues. For DATUM gateway the config would look like:

httpnotify=http://localhost:7152/NOTIFY

I propose that this operate as a "single-shot" system, with no retries. Either the receiver is ready to accept the notification, or they lose it. I propose that only HTTP is supported, and that this is done without any external libraries, using socket/send/recv/close. I propose using this approach because it is very lightweight, with minimal code ripple.
I propose that nothing more be done with errors except log them.

Because this would be a small change, would this be a good first project for people working on Bitcoin Knots?

[luke-jr]

There's a half dozen *notify options, it doesn't make sense to call this one httpnotify...

Maybe just blocknotify=http://... and look for the http: prefix?

[pdath]

I like it. I like it a lot.

My first preference would be to have someone who knows what they are doing implement this ...

If no one wanted to pick this up, I would have a go. I am familiar with C, but not C++. I expect this to be a small code ripple, so it would be a good introductory change.

Your thoughts?