fix: replace boilerplate code and consolidate common classes

[akafredperry]

closes #323

- What I did

• Removed "boiler plate" Java code where possible
• Converted Key classes and Metadata class to immutable classes.
• Consolidated KeyStringUtil.java and KeyBuilders.java into Keys.java
• Builders are now always "fluent api" style
• Improved preconditions (so that builders throw exceptions rather than generate invalid results)
• Improved Unit test coverage (the classes I touched no longer rely on end to end tests for coverage)
• Addressed thread safety of Metadata changes

- How I did it

• Metadata is now an immutable class with private fields. None of the fields have defaults anymore. The getter methods use the "fluent api" style. There is a corresponding builder class MetadataBuilder. Metadata has a toBuilder() method to help creating modified metadata instances and the static method toMergedBuilder replicates the squash() for combining the fields of two metadata instances. Where possible Lombok has been used to remove boiler plate code.
• The abstract class AtKey is now immutable (other than the metadata field) with private fields. The getter methods use the "fluent api" style. There are two methods for updating the medata field. One which will update unset fields and one which will overwrite. Each of the subclasses now has a corresponding builder and this is the only way to construct instances. These are all Lombok builders "backed by" a static method. These replace KeyBuilders.java. Additionally there is a builder that will construct AtKey subclasses from a "raw key" (the fully qualified key string), this replaces KeyStringUtil.java.
• VerbBuilders have been completely re-written to use Lombok builders ("backed by" static methods). This class covers all command string composition in at_java and any code that was previously composing comand strings directly now uses the appropriate builder.
• Atsign is now a Typed String class
• "Well known" key names have been consolidated in AtKeyNames.java

NOTES: #363 collates suggestions on further refactoring which should be done separately from this issue (to help review)

- How to verify it

• Runs the unit and integration tests

mvn clean install

- Description for the changelog

make Keys and Metadata immutable and remove boiler plate code

[gkc]

LGTM thank you @akafredperry