Bug fix: attempt to avoid hangs when creating blockstores on large SSDs.
At least I think the problem is that we are doing inadvertent TRIM operations on large (480GB) SSDs. For one, when creating an ext4 filesystem on such a blockstore, we specify "nodiscard". I toyed with the idea of turning off "issue_discards" for the lvremove operations when a blockstore is destroyed, but that led to old metadata being seen when the blockstore was re-created. That led to the last change, which was to force metadata zeroing when we do an lvcreate of a blockstore.
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