MINOR: stats: report correct throttling percentage for servers in slowstart

MINOR: stats: report correct throttling percentage for servers in slowstart

The column used to report the throttle percentage when a server is in
slowstart is based on the time only. This is wrong, because server weights
in slowstart are updated at most once a second, so the reported value is
wrong at least fo rone second during each step, which means all the time
when using short delays (< 20s).

The second point is that it's disturbing to see a weight < 100% without
any throttle at the end of the period (during the last second), because
the effective weight has not yet been updated.

Instead, we now compute the exact ratio between eweight and uweight and
report it. It's always accurate and describes the value being used instead
of using only the date.

It can be backported to 1.4 though it's not particularly important.