Validate that to/from DIDs are the expected values across an exchange

[phoebe-lew]

When Alice submits an Order or Close, the from DID value should be the same as the initial Rfq from value. An error should be returned if it is different as the DIDs don't match.

This could be a utility function in http-server or generalized in protocol, where the function also checks that all messages sent by the PFI have a consistent DID.