Store the raw value of the ETag received from the server

Right now we parse them as integers though the ETag should remain an opaque value, and so we should store it as we receive it, aka an opaque string.

Note: that would break backward compatibility, hence tagged as 2.0.

The issue names no source file, entry point, or test. Start by locating where server ETag values are parsed and stored, then trace the related synchronization tests. Done means preserving the received ETag as an opaque string and updating coverage for the changed representation, with backward-compatibility impact considered for the 2.0 release.

Written by the indexing model from the issue text.