MINOR: log: "drop" support for log-profile steps

MINOR: log: "drop" support for log-profile steps

It is now possible to use "drop" keyword for "on" lines under a
log-profile section to specify that no log at all should be emitted for
the specified step (setting an empty format was not sufficient to do so
because only the log payload would be empty, not the log header, thus the
log would still be emitted).

It may be useful to selectively disable logging at specific steps for a
given log target (since the log profile may be set on log directives):

log-profile myprof
  on request format "blabla" sd "custom sd"
  on response drop

New testcase was added to reg-tests/log/log_profiles.vtc