MINOR: checks: Add support of payload-based sample fetches

MINOR: checks: Add support of payload-based sample fetches

It is now possible to call check.payload(), check.payload_lv() and check.len()
sample fetches from any sample expression or log-format string in a tcp-check
based ruleset. In fact, check.payload() was already added. But instead of having
a specific function to handle this sample fetch, we use the same than
req.payload().

These sample fetches act on the check input buffer, containing data received for
the server. So it should be part of or after an expect rule, but before any send
rule. Because the input buffer is cleared at this stage.