MEDIUM: spoe/tcpcheck: Reintroduce SPOP check as a customized tcp-check

MEDIUM: spoe/tcpcheck: Reintroduce SPOP check as a customized tcp-check

To be able to retrieve accurrate errors when a SPOP health-check is
performed, a customized tcp-check is used. Indeed, it is not possible to
rely on the SPOP multiplexer for now because the check is performed at the
mux connection layer and the error, if any, cannot be retrieved by the
health-check. A L4 success or error is reported.

To fix this issue and restore the previous behavior, a customized tcp-check
is created. The connection is forced to use the PT multiplexer. An hardcoded
message is sent and a customer handler is used to decode the SPOA response.
This way, it is possible to parse the response and return an accurrate
status code.