improves Stakes::activate_epoch performance (#24068)
Tested with mainnet stakes obtained from the ledger at 5 recent epoch
boundaries, this code is ~30% faster than current master.
Current code:
epoch: 289, elapsed: 82901us
epoch: 290, elapsed: 80525us
epoch: 291, elapsed: 79122us
epoch: 292, elapsed: 79961us
epoch: 293, elapsed: 78965us
This commit:
epoch: 289, elapsed: 61710us
epoch: 290, elapsed: 55721us
epoch: 291, elapsed: 55886us
epoch: 292, elapsed: 55399us
epoch: 293, elapsed: 56803us
fa7eb7f30c