Fetch time at start of routing

[TheBlueMatt]

At #1789 (comment) @jkczyz discovered that (on macOS, but less so on Linux, probably also iOS) our router ends up spending a lot of time fetching the current time to figure out how to decay scorer bounds. At some point we need to either (a) create a "scorer context" that gets created at the start of a route-finding pass to cache the current time or (b) add a timer tick to the ProbabilisticScorer to give it the current time and do the decay'ing outside of route-fetching.

[TheBlueMatt]

We ultimately need to decide between doing this and simply moving to decaying the bounds on a timer, similar to the timer we have for the network graph. In any case, when we do so, we should look again at #2466 (comment)