The `block_contexts` table has a primary key of all the columns, but for
the unencrypted case only three have values, and this doesn't seem to
reject duplicate unencrypted contexts.

Add a separate UNIQUE INDEX for unencrypted block context rows (namely,
no nonce and tag), and change the INSERT to INSERT OR IGNORE to elide
duplicate rows.

…esp/crucible

The `block_contexts` table has a primary key of all the columns, but for
the unencrypted case only three have values, and this doesn't seem to
reject duplicate unencrypted contexts.

Add a separate UNIQUE INDEX for unencrypted block context rows (namely,
no nonce and tag), and change the INSERT to INSERT OR IGNORE to elide
duplicate rows.

Submit different block data as part of the IO loop. Add an option to
submit all zeroes instead if that workload is still desired.

…fwork

This reverts commit 98215b7.

This reverts commit a0daa8b.

This reverts commit e4a2f4b.

This reverts commit 87ae64c.

This reverts commit 5734060.