VDB-1137: Statediff check

[elizabethengelman]

In this release, the statediff.watchedaddresses flag will make sure that the statediff service only processes statediffs for given contract addresses. However, there is currently no filter to make sure that the statediff Payload is not empty before sending it over the subscription. The resulting behavior is that for all blocks that don't have a state diff for the given watched contract address, we end up sending a payload with an "empty" StateDiffRlp:

{ 
   BlockNumber: 60670 
   BlockHash:[52 100 238 122 70 68 56 0 8 232 58 201 18 35 163 192 8 15 70 129 97 112 179 25 60 45 208 183 232 215 212 205]
   CreatedAccounts:[]
   DeletedAccounts:[]
   UpdatedAccounts:[]
   encoded:[]
   err:<nil>
}

and see a lot of sending state diff payload to subscription... logs, which is misleading.

This issue has been fixed in the updated approach to statediffing: #5. Not sure if it makes sense to put in the time to fix the issue here in this statediffing approach as well or just wait until we start using the updated approach. 🤔

[rmulhol]

Definitely on board with adding it to the docs, and creating a story to deal with it later (especially if we build out the diffing service further)

[elizabethengelman]

New story to deal with filtering out empty payloads before sending to subscribers: https://makerdao.atlassian.net/browse/VDB-1157