Add events for withholding node rewards

[joshuahannan]

Closes #446

Adds two events to indicate when nodes are marked to get their rewards withheld and when the withheld rewards are actually calculated

[vishalchangrani]

should we include a ReasonCode? and then have a table either on chain or just in the docs to begin with which has the mapping of
code to reason.
e.g.
ReasonCode 1 - "Node was down for more than 80% of time during the last epoch"
ReasonCode 2 - "Node not updated within 24hr of the Height coordinated upgrade"
Eventually we have many more reasons e.g. incorrect signature on vote for consensus nodes.

[joshuahannan]

That is a good idea. I will probably need you to create that page in the docs site though because you'll know those better than I do. Once that page is on the docs site, I can add a link to it in a comment in the contract.

Do we think just having the reason code as a UInt8 is okay?

[vishalchangrani]

That is a good idea. I will probably need you to create that page in the docs site though because you'll know those better than I do. Once that page is on the docs site, I can add a link to it in a comment in the contract.

Do we think just having the reason code as a UInt8 is okay?

i don't think we will have more than 2^8 (256) reasons so UInt8 sounds good.
Will create a doc site page.