Mapper API and simple implementation.

Design Mapper API for user POJO classes for use in KvView and RecordView.

NB: Most likely, there is no need to have different interfaces for Key/Value/Record mappers
and the same implementation can be reused for all cases.

Let's

• Implement an IdentityMapper as a default one that binds the POJO field to the column with the same name.
• Implement a simple mapper that allows binding the POJO field to the column with a different name.
  Field <--> column mapping expected to be one-to-one function.