CDK-141: Fields that can be incremented with HBase's atomic increment or Accumulo's
aggregators need to have a special anntoation.

Added new mapping type called counter. Only int or long avro fields can be annotated
with this mapping type, and only fields annotated with this mapping type can be
incremented atomically.

Added a new set of schema validations for verifying the following:

1. An entity schema can't contain multiple occVersion mapping fields
2. An entity schema can't contain both an occVersion field and a counter
   field.