Enabling LUKS-backed PVs on lizard takes ages in the initramfs

Originally created by @intrigeri on #12589 (Redmine)

Notices this today after upgrading to Stretch: it took several minutes after each PV unlocking. Each time, 4 pvscan processes were running for a while.

Random ideas:

• Maybe that’s because the initramfs is looking for the nested PVs (PV-on-LV) we have? They’re painful anyway, maybe we should get rid of them.
• Maybe there’s a timeout somewhere that we should lower.

Parent Task: tails/sysadmin#12160

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