refactor walkers

[elliVM]

• Split ConditionWalkers conditions generated from Element into own classes
• Use Interface QueryCondition to get Condition from the new condition classes
• Fix generics in XmlWalker
• Unit tests for new classes
• Equals method and equality tests

[eemhu]

Should there be unit tests for each of the new condition objects?

[elliVM]

added missing tests, IndexStatementCondition test will be updated in a later PR with bloom filter changes.

[elliVM]

Refactored tests they should be cleaner now @eemhu

[eemhu]

I think the changes look good, however I think object equality tests should be added. Also not 100% sure about the use of Optional, it might be considered better practice to use some sort of "empty" condition object instead.

[elliVM]

Added override for equals(Object o) method for conditions and added equality tests. Refactored Optional to use jooq noCondition.

[eemhu]

Good changes, however I would separate them into equality and non-equality tests to make it more clearer. Each test should test only one thing

Also, you could check for this == other as the first statement in equals() but I'm not sure if it does make a meaningful difference.

[elliVM]

No harm in checking for this == other so added and split tests into equalsTest and notEqualsTest