Add support of INSERT queries in PXF JDBC plugin:
- Implement `WriteAccessor` and `WriteResolver` interfaces. Both are implemented in the same classes as `ReadAccessor` and `ReadResolver`;
- Support query batching in Accessor. The size of a batch is defined by user and may be "infinite";
- In Accessor, use `java.sql.PreparedStatement` and built-in functions in it to process queries;
- In `setFields()` method of Resolver, perform type conversions of the data tuples received from PXF.
Optimize and refactor the code in PXF JDBC plugin, make some fixes:
- Make functions for building WHERE statements static where possible to reduce the number of InputData checks;
- Organize imports;
- Check the codestyle;
- Fix the handling of TIMESTAMP values when performing SELECT requests.
- Correct or rewrite Javadoc strings for plugin functions;
- Rewrite README.md.