Coordinate log_line_prefix options 'm' and 'n' to share a timeval.

Coordinate log_line_prefix options 'm' and 'n' to share a timeval.

Commit f828654e introduced the 'n' option, but it invoked
gettimeofday() independently of the 'm' option. If both options were
in use (or multiple 'n' options), or if 'n' was in use along with
csvlog, then the reported times could be different for the same log
message.

To fix, initialize a global variable with gettimeofday() once per log
message, and use that for both formats.

Don't bother coordinating the time for the 't' option, which has much
lower resolution.

Per complaint by Alvaro Herrera.