Fix sonar warnings in sal-distributed-datastore

These come from squid:

- String literals should not be duplicated
- Modifiers should be declared in the correct order
- Lambdas and anonymous classes should not have too many lines
- Nested blocks of code should not be left empty
- Local variables should not shadow class fields
- Exception handlers should preserve the original exception
- Utility classes should not have public constructors
- Overriding methods should do more than simply call the same method in the super class
- Unused private fields should be removed

I fixed quite a few of them. Others we'd have to suppress or modify the sonar
config to be more lenient.

Change-Id: I7ce7b2a05feac9844fd9c37927de82b7b8b68ee5
Signed-off-by: Tom Pantelis <tompantelis@gmail.com>