add a faster mode to find_ids_in_ranges
add a faster mode to find_ids_in_ranges

:loose mode will return ranges that contain at most X ids
rather than a strictly equal set

make it the default for whole table scans (mainly datafixups)

Change-Id: Ic2c15c748749ded69637fda1d90707ba455aef2a
Reviewed-on: https://gerrit.instructure.com/93379
Tested-by: Jenkins
Reviewed-by: Jeremy Stanley <b3f594e10a9edcf5413cf1190121d45078c62290@instructure.com>
Reviewed-by: Cody Cutrer <c4fe2b1d90ef8f2548c8aeabfa633434316f0305@instructure.com>
Product-Review: James Williams  <3a5f4ed991dbfa5c9ef76ad729d240ca12f62180@instructure.com>
QA-Review: James Williams  <3a5f4ed991dbfa5c9ef76ad729d240ca12f62180@instructure.com>

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