Too many queued stakings can cause slow block production

[hallazzang]

We can put ending timestamp inside store keys to mitigate this problem.
When queuing staking, the end time is set to current blocktime + time.Duration(params.NextEpochDays), and queued staking will be processed at the end time, not when the epoch increases.

Tasks

• Fix QueuedStaking logics
• Fix relevant test cases
• Add migration.go, legacy code
• bump ConsensusVersion of farming to 2 from 1
• Update Spec, docs, cli help