OGM-529 Remove OgmIdentifierGeneration

  The "identity" strategy type for primary key generation is not
  correct at the moment. This commit delete the existing strategy and
  fallback to the OgmTableGeneration strategy.
OGM-529 Create sequences at startup

  * Update the tests
  * Apply SEQUENCE label
  * Cache properly
  * Create unique constraint to avoid duplicated node for the same
    sequence
  * Sequences have a 'sequence_name' and 'current_value' properties
OGM-529 Clean up, remove unecessary constants
OGM-529 Add javadoc to NodeLabel