[CSE-246] Add more data constructor
[CSE-246] Add more data constructor

to `Subscription` + `ServerEvent` to handle updates of last page of epochs

input-output-hk/cardano-sl,input-output-hk/pos-haskell-prototype,input-output-hk/pos-haskell-prototype,input-output-hk/cardano-sl,input-output-hk/cardano-sl,input-output-hk/pos-haskell-prototype,input-output-hk/cardano-sl