[META] x/provider

[julienrbrt]

• Manages consumer chains addition/deletion via governance (related Dynamically add consumers #5)
• Create module account: pool linked to each chain, paying fees and rewarding validators (incentive)
• Use new ABCI method to tell consensus / multiplexer about the active consumer chains (related Dynamically add consumers #5)
• Aggregate AppHash from all consumer chains in x/provider (ref App Hash contagion #1). Slash for downtime when necessary (Slashing mechanism #2)

[julienrbrt]

Phase 0: (@julienrbrt)

Spec new ABCI call:

• Must return list of active chains for the new block (similar to validator updates)
• Must aggregate the app hashes of all customer chains in the previous block

Wire this ABCI call to be used by modules (similar to EndBlock with a different signature).
This new ABCI call can be an extension interface, only the provider needs to use it.

Phase 1: (@julienrbrt)

• Create the basic x/provider module that holds state about all consumer chains (name, chain_id, version...)
• Create basic params (fees per block, downtime slashing window)
• Add governance messages: MsgAddCustomer (consumer, block_height) , MsgSunsetCustomer (consumer, block_height), MsgUpgradeCustomer (consumer, version, block_height)
• Create a module account per consumer chain

Phase 2:

Update multiplexer to make use of this new ABCI call. Only use the returned chains to produce blocks.

Phase 3: (@Pantani - #12)

Update x/provider to take fees per block from each chain module account.
If there is too little fees that chain should not be returned from the active chain set.
/!\ We must verify if blocks gaps are ok.

Phase 4:

Implement slashing mechanism based on downtime on customer chain (based on app hash aggregation)

[julienrbrt]

Phase 1 completed as per 21fd2f0, 45dcd4c, 6ee636d, 5d9b624