Snapshot sync and block gap info in eth_syncing

[rphmeier]

Closes #2939.

Adds fields to sync info structure returned from eth_syncing.
These fields are null when irrelevant.
blockGap: array of "first", "last", such that [first, last) are all missing from the chain.
warpChunksAmount: total amount of snapshot chunks.
warpChunksProcessed amount of snapshot chunks processed.

@jacogr All yours!

[jacogr]

Just a basic question - in light of https://github.com/ethcore/parity/issues/2969 do we really want to extend the standard method?

[gavofyork]

@rphmeier can't build (a couple of warnings by the looks of it)

[gavofyork]

@jacogr yeah it's fine - we can do it in one big batch.

[rphmeier]

@gavofyork Yeah, just a test I forgot to update. I'll add serialization tests for the new fields as well.

[rphmeier]

@tomusdrw will the new object format break the old UI?

[gavofyork]

old UI? i don't think there's any of that left, is there?

[tomusdrw]

Yes, old UI is gone (maybe just chrome extension left), but we were never relying on that method anyway.

Since the change is additive it shouldn't also break any web3-based dapps (i.e. Mist)

[coveralls]

Changes Unknown when pulling 009dde4 on snapshot-syncing-rpc into * on master*.

[coveralls]

Changes Unknown when pulling 70fc7cb on snapshot-syncing-rpc into * on master*.

[coveralls]

Changes Unknown when pulling 70fc7cb on snapshot-syncing-rpc into * on master*.

[rphmeier]

The Old UI will still exist in the 1.3.x series. I'd heard it was broken by previous additive changes to RPC, just keeping an eye out. Are we officially taking the position that the old UI is completely unsupported?

[gavofyork]

that is right.

[gavofyork]

travis passing, so will assuming is good.