Add support for subscriptions

[ethanfrey]

We should expose some sort of subscription listener to the RestClient and use it in the CosmWasmConnection. In particular, for full bcp-compatibility, we need to implement:

• watchBlockHeaders
• watchAccount

And postTx could be done nicer with a subscription that with a poll.

I believe these are needed for dropping in CosmWasmConnection into the iov-wallet

[ethanfrey]

Apparently it is not possible to subscribe with the rest server, only via tendermint rpc. Got this response from a lunie wallet dev:

It is not possible to subscribe via the rest server. (frequently requested feature by me :D).

[webmaster128]

Let's just do polling like in the Lisk connector. That brought us quite far and is way more stable than websockets.

[ethanfrey]

Okay

[webmaster128]

What would also work is a hybrid, where you use a tendermint socker subscribtion to get notified about new data. When you receive a new transaction, you ignore the amino data and pull via REST.

[ethanfrey]

Yes, I like this. I think we did some of this at iov. Just have a stream of "update happened" from the subscription service. And those events trigger a rest call to get the data.

I can look into proper tendermint subscription queries, but that would also require a second url and connection (maybe optional?)

[webmaster128]

For wallet integration we need watchAccount and liveTx. The first one is easy. The second one can be implemented like Lisk. This is horribly inefficient without min/max height search filters, but those will be available soon.

[webmaster128]

Closing since the BCP implementation is moving out of here and subscriptions will get their comeback in a couple of weeks when we successfully communicate with Tendermint.