MINOR: sample: accept_date / request_date return %Ts / %tr timestamp values

MINOR: sample: accept_date / request_date return %Ts / %tr timestamp values

Implement %[accept_date] which returns the same as %Ts log-format tag.
Implement %[request_date] which is a timestamp for %tr.

accept_date and request_date take an faculative unit argument which can
be 's', 'ms' or 'us'.

The goal is to be able to convert these 2 timestamps to HAProxy date
format like its done with %T, %tr, %trg etc