Add blockchain.outpoint.subscribe RPC

[romanz]

Following #444 (see the specification here).

Blocked until Electrum client is released with support of 1.5 protocol.

[Pantamis]

Code looks very good to me (much better than mine in #446 obviously) and I successfully use it with janoside/btc-rpc-explorer#356

This "test" doesn't use the notification part however !

[romanz]

Thanks for testing!

[Pantamis]

Would it be possible to rebase this PR with current p2p branch ?

I would like to try #468 with my PR in the bitcoin explorer janoside/btc-rpc-explorer#356 Thanks :)

[romanz]

Thanks for the heads-up! Rebased aec27af.

[Pantamis]

I suppose you will reopen this PR once Electrum 1.5 spec are merged ?

Great work on the p2p branch !

[Kixunil]

I think this was the same GitHub confusion as in #455

[romanz]

I think this was the same GitHub confusion as in #455

Yes, reopening now :)

[Pantamis]

Hi !

Just for information:

This PR still work as expected when cherry picked from master. This is mandatory since #783

The only thing that must be changed to make it compiling is BlockHash::default() to BlockHash::all_zeros() in OutpointStatus struct.

I re tACK this with janoside/btc-rpc-explorer/pull/356