Archiver restart timer

I enclose a short patch to reduce the PGARCH_RESTART_INTERVAL from 60
seconds to 10 seconds. The original number was plucked from thin air
some months ago, and I'd like to review that now based upon further
thought, observation and experience.

This change has little or no effect on performance, since the interval
is there mainly to avoid repeated respawn attempts if archiver fails at
startup. Archiver start-up time is very quick, so there is little danger
of exceeding 10 seconds.

On a busy system, if the archiver does die, then many files can build up
in the 60 seconds before respawning. That xlog file backlog could take
some time to clear. This then leaves a larger than normal window of data
loss for a possibly long period.

It's a minor change only, with no other effect on function.

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Best Regards, Simon Riggs