Handle a pair of request and response.
| req |
request being handled |
| res |
response to send back to the requester |
| next |
continuation to keep routing |
| context |
routing context which parent is the context of the preceeding 'next' invocation or initialized by the first Valum.MatcherCallback |
|
'true' if the request has been or will eventually be handled, otherwise 'false' |
| Error |
callback are free to raise any error |