BLUEBUTTON-1685 Immutable crosswalk

[whytheplatypus]

Change Details

Prevent any change to identity fields of the Crosswalk model after their initial value is set.
This provides confidence that identity is a constant within the system.

Acceptance Validation

• the code is clean and readable
• the tests assert that after the value is set identity fields in the crosswalk won't change

Feedback Requested

• Are the tests enough?
• Is the _ prefix python idiom enough to discourage future devs from accidentally undoing this?
• Are there security concerns>

External References

• BLUEBUTTON-1685

Security Implications

None.

[whytheplatypus]

depends on #771

[switzersc-usds]

LGTM

[dtisza1]

This looks good to me!