Java driver 4 object mapper

Implement the object mapper for Java driver 4.

The goal is to follow the same general principles as the driver 3 mapper: mapped entities are defined via annotated classes, queries with annotated methods that either follow established CRUD patterns (select, save, delete) or provide the query string explicitly. There will be however a few notable differences:

• use annotation processing to generate the code at compile time (instead of reflection in driver 3)

• unify the "mapper" and "accessor" concepts into a single component, tentatively called Dao

See the ticket's subtasks for more details.

There is a java2078 branch in GitHub with a basic infrastructure to generate a main mapper class and DAO implementations (that are empty at this point). It should be used as the target for all pull requests, we'll merge it to 4.x when we are ready to release the mapper.