Rename HTTPTimeout::sClockCount (in clock ticks) to sTime_10ms (in 10ms units).

Note that HTTPTimeout::sClockWidth is no longer used for HTTPTimeout (as
if it's value became a constant of 0.01, the fraction 10ms / 1s).
A new variable, HTTPTimeout::sClockWidth_10ms is used to calculate
sTime_10ms (only).

Also HTTPTimeout::mStalled and HTTPTimeout::mLowSpeedClock changed units
to the same as of sTime_10ms (time since epoch in 10ms units).