This promise plugin was using tail_file(self.amarisoft_rf_info_log) to
get the most recent log entry to check. But tail_file simply return last
line of the log file without checking timestamp of returned entry. So it
could be the case that last line is ages ago, reports "CPRI locked ok"
and then, if there are no more rf_info entries for one reason or
another, the CPRI-locked promise will be holding green despite the fact
that data became stale.

-> Fix it by explicitly checking timestamp of last log entry to be in
expected recent range.

/cc @jhuge, @tomo, @xavier_thompson, @Daetalus
/reviewed-by @lu.xu
/reviewed-on !127