sync: work on full-validation-on-startup. 90% done, disabled.

Change-Id: I5bf062f3b22c2cc41329ff6b23f11198ae543c0f
Add missing moved file from 76ba33b2

Change-Id: Iadbdc7e576c40da11fbf3261f26bdbae5afe7f1b
sync: background validation of src-vs-destination in sync handler

Currently controlled by an environment variable, but will become
a config option + on-demand button in UI in later commits.

Change-Id: I25fa878c9b30cdd713e2859585210eb722092f7b