Clarify and rename `firstActiveDay` semantics

[rickstaa]

We added a new firstActiveDay field to the Broadcaster entity in #168. However due to a patch we had to do in #202 (comment) this now is a bit ambiguous because activeBroadcasters on the Protocol entity only tracks broadcasters that are actively sending fees. We might consider deprecating firstActiveDay in favor of a clearer field such as activationTimestap and lastActiveDay to lastActiveTimestamp, and update the schema and related logic accordingly.

Example

"The date this broadcaster first funded a deposit or reserve, beginning at 12:00am UTC"
firstFundedTimestamp: Int!
"The date this broadcaster first funded a deposit or reserve, beginning at 12:00am UTC"
firstActiveDay: Int! @deprecated(reason: "Use firstFundedDay instead.")