MAJOR: checks: completely use the connection transport layer

MAJOR: checks: completely use the connection transport layer

With this change, we now use the connection's transport layer to receive
and send data during health checks. It even becomes possible to send data
in multiple times, which was not possible before.

The transport layer used is the same as the one used for the traffic, unless
a specific address and/or port is specified for the checks using "port" or
"addr", in which case the transport layer defaults to raw_sock. An option
will be provided to force SSL checks on different IP/ports later.

Connection errors and timeouts are still reported.

Some situations where strerror() was able to report a precise error after
a failed connect() in the past might not be reported with as much precision
anymore, but the error message was already meaningless. During the tests,
no situation was found where a message became less precise.