logs-show: get timestamp and boot ID only when necessary

logs-show: get timestamp and boot ID only when necessary

Previously, get_display_timestamp() is unconditionally called even if we
will show logs in e.g. json format.
This drops unnecessary call of get_display_timestamp().

This also makes journal fields in each entry parsed only once in
output_short(). Still output_verbose() twice though.

This should improve performance of dumping journals.

Replaces #29365.

Co-authored-by: Costa Tsaousis <costa@netdata.cloud>