DP: Document v-ops presence/absence when streaming

[relativityboy]

@inertia186 commented:

As an application developer, I want to be able to stream virtual ops from head_block_num instead of last_irreversible_block_num, because I want a live view of what's happening.

head_block_num does not have v-ops.

AC

• document in the DP that this is not the case
• document on streaming tutorial that "currently" while streaming from head is possible, it's an exceptionally bad idea because you lose v-ops (phrase appropriately). Comment that this may change in future (allude to bug, or link to bug)
• admonish against use of head_block_num in general

[feruzm]

https://github.com/steemit/devportal-tutorials-js/blob/master/tutorials/13_stream_blockchain_transactions/public/app.js
JS-T: 13

[inertia186]

This appears to be an artifact of jussi.

Streaming vops on head is a problem that is mostly mitigated by adding a 3 second wait when its encountered. On occasion, it takes a bit longer than 3 seconds.

However, it's not just a problem on the head block. I've also encountered it on irreversible blocks, very rarely.

For my client (steem-ruby), I've added a check when streaming vops, that if it encounters a response of empty vops, it waits 3 seconds and retries. This can cause the stream to fall behind. But it can always catch up by requesting blocks with json-rpc-batch, so it's no big deal.

[inertia186]

Seems related: steemit/steem#2690

[inertia186]

Additional strategy in HF20: steemit/steem#2099 (comment)