Allow to merge deactivating stake accounts in the same epoch

[marcin-cb]

Currently Staking on Solana, one can merge activating stake accounts in the same epoch that have the same stake authorities and stake to the same vote account.

However the opposite is not true. That is, the Stake program does not allow to merge deactivating stake accounts.

It'd be great to have this feature implemented if it's not too complex and if it doesn't break any existing assumptions.

[2501babe]

i dont have strong opinions on this either way, it could be nice if it makes the code simpler, but it would be a substantial refactor. for now, we arent doing any feature work on the stake program until the bpf version launches on mainnet, and then i have a few fixes/improvements for the first post-launch release. will leave this open and revisit the idea in the future