Insufficient asset quota and deposits

[gavofyork]

Fixes #10378
Polkadot companion: paritytech/polkadot#4390
Cumulus companion: paritytech/cumulus#804

Make two changes:

• System accounts can only have a maximum number of consumers; once this limit is exhausted, any consumer-requiring operations will fail. This includes new asset-accounts.
• Asset accounts (i.e. the storage of a specific account's balance of a specific asset) can now have three reasons for existing: the old two (i.e. the asset being "sufficient" or taking up a consumer reference) as well as having a deposit explicitly held for the asset-account. This deposit is placed by using the new touch function and refunded by using the new refund function.

Additional considerations

Consider the possibility of requiring touch to be used to receive any insufficient asset, which would mean the user would have to take active measures to receive an asset but that they could not gather consumer references inadvertently.

Downstream Code Migration

The System config trait frame_system::Config now has an additional item MaxConsumers. e.g.

impl frame_system::Config for Runtime {
  /* snip */
  type MaxConsumers = frame_support::traits::ConstU32<16>;
}

The Assets config trait frame_assets::Config now has an additional item AssetAccountDeposit. e.g.

parameter_types! {
  pub const AssetAccountDeposit: Balance = 5 * CENTS;
}

impl frame_assets::Config for Runtime {
  /* snip */
  type AssetAccountDeposit = AssetAccountDeposit;
}

[adoerr]

beefy-mmr, beefy and merkle-mountain-range LGTM

[girazoki]

The impression I have is that the new ExistenceReason enum is designed to avoid migrations with the current boolean type right? So my assumption is that the PR does not imply data migrations?

[gui1117]

the trait FrozenBalance says:

	/// In reality, the balance of every account must be at least the sum of this (if `Some`) and
	/// the asset's minimum_balance, since there may be complications to destroying an asset's
	/// account completely.
	///
	/// If `None` is returned, then nothing special is enforced.
	///
	/// If any operation ever breaks this requirement (which will only happen through some sort of
	/// privileged intervention), then `melted` is called to do any cleanup.

I think it needs to be updated.

[gavofyork]

@thiolliere @shawntabrizi @KiChjang Updated.

[shawntabrizi]

looks good to me now

[gui1117]

the trait FrozenBalance says:

	/// In reality, the balance of every account must be at least the sum of this (if `Some`) and
	/// the asset's minimum_balance, since there may be complications to destroying an asset's
	/// account completely.
	///
	/// If `None` is returned, then nothing special is enforced.
	///
	/// If any operation ever breaks this requirement (which will only happen through some sort of
	/// privileged intervention), then `melted` is called to do any cleanup.

I think it needs to be updated.

@thiolliere How so?

It is now good with #10431

[gavofyork]

@thiolliere An approval would be good :)

[gui1117]

looks good to me