BUG-5626: use Identifier instead of String

Using a String for identifying replication entries is not flexible
enough. Yangtools has an Identifier concept and utility classes that
make this more flexible. Use these so we can evolve the identifiers
we use to track requests.

Change-Id: Ie5794d1e929300928c57cbec6d2fe22329fe6a5e
Signed-off-by: Robert Varga <rovarga@cisco.com>