RPC: rewards, return error if epoch_boundary_block is a lie

[CriesofCarrots]

Problem

In getInflationReward, the epoch_boundary_block determined here may not actually be the epoch boundary block. This can happen if there is a gap in the ledger or long-term storage followed by finalized blocks.
In such a case, it will be impossible to find any rewards.

Summary of Changes

Check the parent slot of epoch_boundary_block; if it is not smaller than the known first_slot_in_epoch, epoch_boundary_block is a lie. Return a custom RPC error in this case, pointing to server-side issues.

[mergify]

If this PR represents a change to the public RPC API:

1. Make sure it includes a complementary update to rpc-client/ (example)
2. Open a follow-up PR to update the JavaScript client @solana/web3.js (example)

Thank you for keeping the RPC clients in sync with the server API @CriesofCarrots.

[mergify]

Backports to the beta branch are to be avoided unless absolutely necessary for fixing bugs, security issues, and perf regressions. Changes intended for backport should be structured such that a minimum effective diff can be committed separately from any refactoring, plumbing, cleanup, etc that are not strictly necessary to achieve the goal. Any of the latter should go only into master and ride the normal stabilization schedule. Exceptions include CI/metrics changes, CLI improvements and documentation updates on a case by case basis.

[t-nelson]

logic lgtm. left one error message clarifying nit that you can take or leave

[mergify]

Backports to the stable branch are to be avoided unless absolutely necessary for fixing bugs, security issues, and perf regressions. Changes intended for backport should be structured such that a minimum effective diff can be committed separately from any refactoring, plumbing, cleanup, etc that are not strictly necessary to achieve the goal. Any of the latter should go only into master and ride the normal stabilization schedule.