bond EPIC: x/sequencers cleanup unbonding / jail / slash

[danwt]

With recent work relating to sequencer rotation, bond increase/decrease and jail and slash changes, the sequencer module needs to be improved.

This issue is a meta issue tracking other sub issues

Nits

• x/sequencers: fix bond reduction docstrings  #1056
• x/sequencers: fix var name  #1057
• x/sequencers: split sequencer unbond and bond reduction concepts #1062

Important

• x/sequencers: fix reduce bond if condition  #1058
• x/sequencers: simplify bond update allowed to take sequencer #1059
• x/sequencers: bond reduction min bond check remove callsites #1064
• x/sequencers: dry unbonding code #1066
• x/sequencers: jailed check on bond reduction is not needed #1067
• x/sequencers: use MustGetSequencer #1068
• x/sequencers: document and fix insufficient bond check in bond reduction completion #1069
• x/sequencers: start sequencer unbonding when they are jailed #1071
• x/sequencers: document reason for only bonded sequencer able to reduce bond #1072

Critical

• x/sequencers: unbond reduction import / export genesis #1060
• x/sequencers: define invariants around unbonding reductions #1061
• x/sequencers: check jail not imply 100% slash assumption #1063
• x/sequencers: fix DOS/inefficiency in getSequencerDecreasingBonds #1065
• x/sequencers: check IsAllLT coins usages in unbond logic #1070
• Incorrect Coins comparison allows decreasing sequencer bonds below the configured minimum #1255
• Flaws in the unbonding process allow for instant withdrawals #1257

Urgent/Blocking

• x/sequencers: fix linter and tests since bond reduction PR #1073

[danwt]

I've assigned @spoo-bar but there are relevant things for myself and @mtsitrin (#1071) (#1066)

[danwt]

superseded by #1328