Implement eth_subscribe('syncing')

[amaury1093]

Didn't find this in the issues, so posting it here.

Before filing a new issue, please provide the following information.

• Parity Ethereum version: 2.3.0
• Operating system: MacOS
• Installation: homebrew
• Fully synchronized: yes
• Network: ethereum / ropsten / kovan / all
• Restarted: yes

Repro:
create a WS connection and send {"id":2,"jsonrpc":"2.0","method":"eth_subscribe","params":["syncing"]}

Expected:
works

Actual:

{"jsonrpc":"2.0","error":{"code":-32000,"message":"This request is not implemented yet. Please create an issue on Github repo."},"id":2}

[Tbaut]

This issue got spotted thanks to Fether but it also happens on full nodes.

[tomusdrw]

It was intentionally not implemented, cause the definition of "syncing" is a bit vague :) I guess we should make it work using is_major_sync heuristic: on every block import check is_major_importing and trigger the notification if the flag flips.