Rework reward caching

[jacogr]

Currently we get this (when the user has not visited for a while) -

The drop-off there on 342 is because of an incomplete session. We should rather track to make sure we have rewards for all era-ends. (Bit tricky generally, since not all epoch ends are era ends as per current.)

So probably need to add a flag to make sure it is indeed session-ending. If not, discard.

Currently we always re-load at least a full era, which is wasteful on each load - and well, if you stay . away fro 2 days, you have issues.

[polkadot-js-bot]

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