Archival support without requiring a separate gw instance

[Yawning]

nb: Filing this here, though this probably needs changes in https://github.com/oasisprotocol/oasis-sdk

The archive node support as implemented in #281 is sub-optimal in that it uses a dedicated web3 gw instance to handle queries that hit up the archival oasis-node instance. This is because oasis-node's grpc interface is not guaranteed to be stable and in fact has changed in a binary incompatible way.

As it is essentially impossible to import multiple versions of go packages in a useful manner (No, replace in go.mod does not work), the decision was made to use an archival web3 gateway instance that the non-archival web3 gateway instance will query to service 5 calls.

While there is (untested) support for having web3 gateway instances share the backing database, in an ideal world, a single web3 gateway instance will be able to handle both current and archival queries.