Add --unsafe-skip-upgrade CLI flag

[aaronc]

Context

There are scenarios in which a planned upgrade handled by the upgrade module needs to be aborted. This could happen when:

• an error happens in the upgrade handler in the new binary and the upgrade can't complete
• there is a problem with the upgrade handler that causes a consensus failure (note that solving this also requires --unsafe-roll-back)

Acceptance Criteria

Given that an upgrade needs to be aborted because the new binary simply doesn't work right
When a user invokes the old binary with the --unsafe-skip-upgrade flag (ex. xrnd start --unsafe-skip-upgrade)
Then old binary will continue to operate as before and clear the planned upgrade from state

[ethanfrey]

If flag is present, we must mark the current plan as done before https://github.com/cosmos/cosmos-sdk/blob/9652ec5a504c12019ce62e8b90d7fd8676d4610f/x/upgrade/abci.go#L23-L24

Marking as done is easy, once you have the Keeper: https://github.com/cosmos/cosmos-sdk/blob/9652ec5a504c12019ce62e8b90d7fd8676d4610f/x/upgrade/internal/keeper/keeper.go#L73-L78

[ethanfrey]

You need the upgrade Keeper and access to cli flags, not sure where the best place to do so is - app init or in upgrades/BeginBlock.

Please add your reasoning which is the better place for it

[ethanfrey]

We need to add the flag here: https://github.com/cosmos/cosmos-sdk/blob/9652ec5a504c12019ce62e8b90d7fd8676d4610f/server/start.go#L83-L84

But where do we run the logic from the flag

[ethanfrey]

Please start with a proposal on where to add the logic (if flag set, clear keeper plan) and why this is the best location

[sahith-narahari]

I think it should go to upgrade/beginblock,as importing from child to parent isn't a good idea.
And how about just returning from upgrade/beginblock rather than clearing keeper plan

[sahith-narahari]

Also, we don't think it's possible to import keeper in init. Instead accessing flag inside BeginBlocker makes sense.

[sahith-narahari]

@aaronc @ethanfrey, please review this solution