Type signature changes for `headerDecoder` and `httpEndpoint`

Type signature changes are described respectively in

• headerDecoder 97fc97b#commitcomment-57041256
• httpEndpoint 59068fb#commitcomment-57041258

This release also updates the generator to include a few new MsgFromClient and MsgFromServer out of the box

• ManyMsgFromClient (List MsgFromClient) and ManyMsgFromServer (List MsgFromServer) variants are convenient to batch several messages to a single request and/or response
• ClientServerVersionMismatch is a new default message that Client can understand that its version is out of sync with Server and should reload (or ask the end user to reload)