Batch ledger accesses when calling User_command.to_verifiable on a bundle of commands

[nholland94]

In order to convert a user command to its verifiable format, verification keys required for verifying parties commands need to be loaded from the ledger. However, in the current implementation, we will do n queries to the ledger for this information rather than 1 since we call to_verifiable in a loop. We should update the code so that we can call to_verifiable in a way such that we can request all of the verification keys from the ledger in batch.