Refactor relocation system with trait-based design

[asonnino]

Suggestion

Convert RelocationWatermark into a trait with default functions:

• relocation_file_path
• load
• save

Create WalRelocationWatermark and IndexRelocationWatermark that implement this trait.

The current RelocatorDriver would take as input a generic Relocator that implements default functions that are common to all relocation strategies.

Create a separate WalRelocator and IndexRelocator implementing Relocator. These structures are respectively initialised with WalRelocationWatermark and IndexRelocationWatermark and only contain the logic of their respective relocation strategy.

[asonnino]

@phoenix-o based on our discussion yesterday I am stopping this refactor until you give me the green light

[andll]

If we do this we need to make sure there is some actual material benefit (e.g. code become easier to read, etc)