Make DIDConn connections uni-directional (one-way delivery path)
A didcomm connection created in one SVDX is not mirrored on the other side of the connection to the other SVDX. This means SVDX1 can have a didcomm connection with SVDX2 and transport mails over DIDComm, but SVDX2 may not have the corresponding connection and may not be able to transport mails over DIDComm to SVDX1. Therefore, connections in SVDX services are independent and self-contained one-way path for mail delivery not related in any way to connections established in other SVDX services.
Closes #15 (closed)