Import benchmarking

[jaffee]

Overview

Adds a series of benchmarks for importing into fragments and then updating them

Fixes #

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[seebs]

Slight lingering concerns about the unchecked possible errors from deferred Clean() calls, but I think this is reasonable as-is. Didn't fully vet every detail, but I checked where timers are started and stopped and things like that, and it looked reasonable.

[seebs]

Looks reasonable to me.