Regions can now be read-only

[jmpesp]

Up until this point, regions were only ever read-write, and region snapshots were read-only. In order to support snapshot replacement, Crucible recently gained support for read-only downstairs that performs a "clone" operation to copy blocks from another read-only downstairs.

This commit adds a "read-only" flag to Region, and adds support for Nexus initializing a downstairs with this new clone option.

[leftwo]

As usual, I have questions because I don't know how anything works

[smklein]

Just to make sure I grok this correctly:

• Nothing actually tells Crucible about this flag yet, right?
• The case of read_only = false is currently treated identically to read_only = true, with the only exception being the error thrown in region_replacement_start.rs, correct?

Did you expect for this flag to have a meaning before merging, or is this acting as more of a placeholder?

[jmpesp]

Nothing actually tells Crucible about this flag yet, right?

Correct, this PR was carved out of the snapshot replacement work to be reviewed separately.

The case of read_only = false is currently treated identically to read_only = true, with the only exception being the error thrown in region_replacement_start.rs, correct?

Yeah - allocation wise there isn't much difference between a read-only region and a read-write region, they both require the same amount of space. The difference is in how they're initialized.

Did you expect for this flag to have a meaning before merging, or is this acting as more of a placeholder?

Right now this is only a placeholder - the snapshot replacement PR number 3 are is the one that will make use of it.