logmsg: add log_msg_set_timestamp() and log_msg_set_timestamp_with_zone_offset()
    functions

Signed-off-by: Tibor Benke <btibi@balabit.hu>
syslogformat: move log_msg_parse_date() into lib/dateparse.h

log_msg_parse_date() is a date parser function only used by syslogformat. From
    now on, it can be used from any other location.

Signed-off-by: Tibor Benke <btibi@balabit.hu>
json-parser: rename _SDATA key to .SDATA, so it can be parsed into the
	nvtable.

Signed-off-by: Tibor Benke <btibi@balabit.hu>
json-parser: when the DATE field is present in the message, it parses its
    content and sets the appropriate value in the logmsg structure.

Signed-off-by: Tibor Benke <btibi@balabit.hu>