OGM-538 Removal or RowKey#getTable() - NOT for inclusion

@DavideD, I experimented with removing `RowKey#getTable()` (only the last commit is important).

It works for all the backends with the exception of Neo4j; I'm getting one assertion failure and two test failures there. Could you take a look at this? I guess I've made some mistake when adapting the Neo4j dialect (generally I replaced may `Key` parameters by `EntityKey`). It might already be resolved by rebasing this onto your work of adding `RowKey#getTargetKey()`.

Thanks!