rate: simplify code

[josharian]

When a long time has passed since a previous call to the rate limiter,
we can overflow when calculating how many new tokens are available.

To prevent that, we calculated a maximum elapsed amount.
When we used maxElapseed, we ended up converting a duration into tokens
and then back. This operation was lossy, so we took pains to increase
the precision of the conversion in tokensFromDuration.

An easier fix is to avoid the round trip entirely.
We know the max number of new tokens we should add; cap it directly.
This lets us use the original, simpler tokensFromDuration.

While we're here, tweak durationFromTokens for clarity.

Updates #34861

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[josharian]

Oops, context switching fail. Will re-send via gerrit later.