Make throttling on tlog disk space more gradual

[tclinkenbeard-oai]

Currently, if a tlog runs out of disk space, throttling ramps up extremely quickly once MIN_AVAILABLE_SPACE_RATIO is reached. This PR introduces a new TLOG_THROTTLE_START_AVAILABLE_SPACE_RATIO knob that supports more gradual throttling.

By default the knob has no effect, but if set to e.g. 0.2 (with MIN_AVAILABLE_SPACE_RATIO at the default 0.05), the target tlog queue size (used for the ratekeeper's tpsLimit calculation) scales linearly downward as available space decreases from 20% to 5%.

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[gridem-openai]

Do we want to have START and STOP as configurable params?

[tclinkenbeard-oai]

STOP is effectively MIN_AVAILABLE_SPACE_RATIO, there isn't a separate knob for this yet, but I don't think we need one

[gridem-openai]

lgtm!

[gxglass]

@tclinkenbeard-oai I'll have a look. I haven't looked at any of this stuff before so need a little time.