Fix RideFileCache threaded db access

The call to RideFile::getWeight() in RideFileCache
computation ends up with a call the read the measures
table in the DB. This is BAD since it is called from
a thread.

We now call getWeight() in the metric aggregator before
launching the ridefilecache computation -- this will cache
the weight and avoid db access.

I've also removed the duplicate code in the w/kg metric code
too and it should be marginally faster at computing metrics
now.

BIG THANKS TO ILJA BOOIJ FOR HIGHLIGHTING THIS, DESPITE MY
INITIAL SCEPTICISM. I OWE HIM A PINT (OR TWO) :)

Fixes #604