Persist activated bundle etag to store

Currently etag from the HTTP response of activated bundles is not
persisted to store. Hence if OPA restarts and an activated bundle
loaded from the disk store is up-to-date, OPA may still download
the same version of the bundle and activate it. With this change,
OPA should include the right etag in the bundle download request
thereby avoiding unnecessary bundle download and activation.

Fixes: #4544

Signed-off-by: Ashutosh Narkar <anarkar4387@gmail.com>