[NPoS] Fix for Reward Deficit in the pool (#1255)

closes https://github.com/paritytech/polkadot-sdk/issues/158.
partially addresses
https://github.com/paritytech/polkadot-sdk/issues/226.

Instead of fragile calculation of current balance by looking at `free
balance - ED`, Nomination Pool now freezes ED in the pool reward account
to restrict an account from going below minimum balance. This also has a
nice side effect that if ED changes, we know how much is the imbalance
in ED frozen in the pool and the current required ED. A pool operator
can diligently top up the pool with the deficit in ED or vice versa,
withdraw the excess they transferred to the pool.

## Notable changes
- New call `adjust_pool_deposit`: Allows to top up the deficit or
withdraw the excess deposited funds to the pool.
- Uses Fungible trait (instead of Currency trait). Since NP was not
doing any locking/reserving previously, no migration is needed for this.
- One time migration of freezing ED from each of the existing pools (not
very PoV friendly but fine for relay chain).

substrate/bin/node/runtime/src/lib.rs

@@ -521,8 +521,8 @@ impl pallet_balances::Config for Runtime {
 	type ExistentialDeposit = ExistentialDeposit;
 	type AccountStore = frame_system::Pallet<Runtime>;
 	type WeightInfo = pallet_balances::weights::SubstrateWeight<Runtime>;
-	type FreezeIdentifier = ();
-	type MaxFreezes = ();
+	type FreezeIdentifier = RuntimeFreezeReason;
+	type MaxFreezes = ConstU32<1>;
 	type RuntimeHoldReason = RuntimeHoldReason;
 	type MaxHolds = ConstU32<2>;
 }
@@ -882,6 +882,7 @@ impl pallet_nomination_pools::Config for Runtime {
 	type WeightInfo = ();
 	type RuntimeEvent = RuntimeEvent;
 	type Currency = Balances;
+	type RuntimeFreezeReason = RuntimeFreezeReason;
 	type RewardCounter = FixedU128;
 	type BalanceToU256 = BalanceToU256;
 	type U256ToBalance = U256ToBalance;