dispatcher: improve comment
dispatch_connection: drop pointless work in cleanup

We no longer are removing a connection slot, we simply flag it as being
available for reuse, so no need to lookup the connection in the
connection table, and possibly find inconsistencies, since they cannot
happen, we got a pointer from an array.
dispatch_connection: make sure we return nothing done

If we haven't done anything (because we couldn't read) make sure we
return 0 to indicate we didn't do anything (useful).  As such the main
loop will correctly slow down for a bit to avoid spinloops.

This might be the cause and solution for issue #8.
Merge https://github.com/grobian/carbon-c-relay