MEDIUM: checks: Support log-format strings for tcp-check send rules

MEDIUM: checks: Support log-format strings for tcp-check send rules

An extra parameter for tcp-check send rules can be specified to handle the
string or the hexa string as a log-format one. Using "log-format" option,
instead of considering the data to send as raw data, it is parsed as a
log-format string. Thus it is possible to call sample fetches to customize data
sent to a server. Of course, because we have no stream attached to healthchecks,
not all sample fetches are available. So be careful.

    tcp-check set-var(check.port) int(8000)
    tcp-check set-var(check.uri) str(/status)
    tcp-check connect port var(check.port)
    tcp-check send "GET %[check.uri] HTTP/1.0\r\n" log-format
    tcp-check send "Host: %[srv_name]\r\n" log-format
    tcp-check send "\r\n"