Mempool: inject transactions from older eras

[edsko]

The HFC should automatically inject transactions from one era into the next in the mempool (parameterized by a function to do so, obviously, between every consecutive pair of eras). The point of doing this is that there is client code out there that submits transactions but does not follow the chain (at least, not directly), but instead uses services such as the REST API to see if their transaction made it -- so those would continue to work even if we move to Goguen, and they haven't upgraded.

[edsko]

Note that it is up to every ledger then to decide whether they want to have an explicit constructor for "transaction from previous era" (with corresponding tag in the binary format) or simply design the ledgers so that transactions from previous eras can be deserialised as-is as transactions from the new era; the consensus/HFC side perspective would be unaffected by this choice.