Fix caching of /detail for modified reviewer lists

Rely only on ETag for change entity caching. The Last-Modified
value was derived from the lastUpdatedOn property, which is not
always modified when children are updated.

When adding or removing reviewers from a change perform a no-op
modification of the Change entity. This bumps its rowVersion as
a side-effect of the database mutation, causing the ETag to be
different on the next read.

Change-Id: If480f92b419041e7d62839ab12211058e78f828f
4 files changed