cache migrations globally closes #7443
cache migrations globally closes #7443

we never add/remove migrations while code is running, and expect to
pick up the changes without restarting the process, so this is fine

test plan: n/a

Change-Id: I8420e99bc32b0eb5b166e870b31d8bdb02a7b164
Reviewed-on: https://gerrit.instructure.com/9010
Tested-by: Hudson <dfb64870173313a9a7b56f814c8e3b33e268497a@instructure.com>
Reviewed-by: Brian Palmer <3cb4e1df5ec4da2c7c4af7c52cec8cf340a55a10@instructure.com>

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