tech debt: cleanup or remove transactions "stacks"

[marta-lokhova]

To be done after #3782 is completed and rolled out on the network.

The change above makes the notion of "tx stacks" (aka chains of transactions for the same source account) obsolete.
At a minimum, we can change the API so callers like the transaction queue and transaction limiter don't need to be aware of the tx stacks used underneath. We could also potentially simplify the underlying surge pricing ordering mechanism to remove the notion of tx stacks completely, but this might be harder due to the need to validate pre-protocol-20 transaction sets, which still contain tx stacks. This requires investigation to determine code to clean up/remove.

[marta-lokhova]

As part of this cleanup, we should also remove any logic inside the TransactionQueue module that deals with chains of transactions per source account.

[MonsieurNicolas]

+1 on doing this work. When we do it, we should also revisit tests as all this complexity is not needed and in some cases the intent of the test may not be valid anymore.

I don't think we need to validate pre-20 txsets: when replaying transactions from history, the only thing we need to ensure is that hashes are correct (we don't call checkValid on tx sets and only apply transactions).