program: use stake history

[2501babe]

currently, the program does not properly account for warmup/cooldown when determining activation status. we should do it the right way, especially now that we are >=2.1 and can get stake history via syscall

[2501babe]

this is easy enough to fix at the same time as #581. we should do it for DepositStake because i noticed a very stupid case where you can deposit to an inactive pool. this has no downsides but we should block it to have less to think about

WithdrawStake allows withdrawals from an inactive pool and this is good and right. add a comment saying so