Ogm 264

(Based on top of PR #336; The actual diff of this PR is [here](https://github.com/gunnarmorling/hibernate-ogm/compare/gunnarmorling:OGM-481...gunnarmorling:OGM-264?expand=1))

* Clearly separates `OgmSequenceGenerator` and `OgmTableGenerator`, also removes many references to SQL from the same
* Establishes new key type `IdGeneratorKey` and passes this to `GridDialect#nextValue()`
* Adds `GridDialect#supportsSequences()`; Depending on the outcome the sequence generator will fall-back to the table generator or not
* Supports both, table and sequence-based strategies on Neo4j; The latter avoids the need for another label with the table name and generally seems a better fit on Neo4j