MEDIUM: checks: Remove dedicated sample fetches and use response ones instead

MEDIUM: checks: Remove dedicated sample fetches and use response ones instead

All sample fetches in the scope "check." have been removed. Response sample
fetches must be used instead. It avoids keyword duplication. So, for instance,
res.hdr() must be now used instead of check.hdr().

To do so, following sample fetches have been added on the response :

  * res.body, res.body_len and res.body_size
  * res.hdrs and res.hdrs_bin

Sample feches dealing with the response's body are only useful in the health
checks context. When called from a stream context, there is no warranty on the
body presence. There is no option to wait the response's body.