runtime: use leader schedule epoch to serve sol_get_epoch_stake

[buffalojoec]

Problem

The bank epoch stakes cache stores stakes according to the leader schedule epoch, which means the epoch stake for a given epoch N actually corresponds to the N + 1 epoch in the epoch stakes cache.

The sol_get_epoch_stake syscall is designed to vend to on-chain programs the epoch stakes for the current epoch, but the existing implementation is returning the epoch stakes for leader schedule epoch N, when it actually should return the epoch stakes for leader schedule epoch N + 1.

For example, if you activate stake in epoch N, it becomes active in epoch N + 1, but the existing bank implementation for serving the sol_get_epoch_stakes syscall actually wouldn't return this new stake until epoch N + 2.

Summary of Changes

Use Bank's current_epoch_stakes method - which fetches the leader schedule epoch bank.epoch() + 1 - to serve the epoch stake information for sol_get_epoch_stakes, properly returning the current epoch stake.

[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.