instrumentation: Keep time fields as instrtime, convert in callers

instrumentation: Keep time fields as instrtime, convert in callers

Previously the instrumentation logic always converted to seconds, only for
many of the callers to do unnecessary division to get to milliseconds. As an
upcoming refactoring will split the Instrumentation struct, utilize instrtime
always to keep things simpler. It's also a bit faster to not have to first
convert to a double in functions like InstrEndLoop(), InstrAggNode().

Author: Lukas Fittl <lukas@fittl.com>
Reviewed-by: Andres Freund <andres@anarazel.de>
Discussion: https://postgr.es/m/CAP53PkzZ3UotnRrrnXWAv=F4avRq9MQ8zU+bxoN9tpovEu6fGQ@mail.gmail.com