MINOR: check: ensure QUIC checks configuration coherency

MINOR: check: ensure QUIC checks configuration coherency

QUIC is now supported on the backend side, thus it is possible to use it
with server checks. However, checks configuration can be quite
extensive, differing greatly from the server settings.

This patch ensures that QUIC checks are always performed under a
controlled context. Objectives are to avoid any crashes and ensure that
there is no suprise for users in respect to the configuration.

The first part of this patch ensures that QUIC checks can only be
activated on QUIC servers. Indeed, QUIC requires dedicated
initialization steps prior to its usage.

The other part of this patch disables QUIC usage when one or multiple
specific check connection settings are specified in the configuration,
diverging from the server settings. This is the simplest solution for
now and ensure that there is no hidden behavior to users. This means
that it's currently impossible to perform QUIC checks if other endpoints
that the server itself. However for now there is no real use-case for
this scenario.

Along with these changes, check-proto documentation is updated to
clarify QUIC checks behavior.