[BUG] checks: don't wait for a close to start parsing the response

[BUG] checks: don't wait for a close to start parsing the response

Cyril Bonté reported a regression introduced with very last changes
on the checks code, which causes failed checks on if the server does
not close the connection in time. This happens on HTTP/1.1 checks or
on SMTP checks for instance.

This fix consists in restoring the old behaviour of parsing as soon
as something is available in the response buffer, and waiting for
more data if some are missing. This also helps releasing connections
earlier (eg: a GET check will not have to download the whole object).