Ogm 264

* 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
It executes the actual key retrieval. But I'm open for other names, do you have a proposal?