Lookup of previous on-chain points

[KtorZ]

Fixes #24

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• 📍 Provide a new database accessor to retrieve checkpoints' ancestors.
  This is built as a simple query on the checkpoints table. The commit
  also add two property test focused on the checkpoints table. It also
  test the insertion and listing of checkpoints (and found a bug). The
  interface is a bit flexible and allows for returning many a list of
  ancestors, even though the main motivating use-case is only about
  single ancestors. It makes for a nicer interface (lists for the win
  o/).

• 📍 Re-organize App.Http, split pure code out and tidy up.
  The module was starting to get big with many level of abstractions
  intertwined. For the sake of keeping the separation good (and
  possibly, indidually improve test coverage of the sub-parts), I've
  move all the pure helpers into separate modules under the 'Data'
  section. Remains in 'App.Http' only the actual route handlers. This is
  a bit cleaner and allow to extend even more the http endpoints while
  keeping things clean and tidy.

• 📍 Add new endpoint to retrive checkpoints by slot number.
  The API is purposely flexible to also make it easy to retrieve
  ancestors from a given point. It suffices to remove 1 from any given
  slot number to get the nearest slot that is smaller or equal to the
  provided slot number. Because this behavior can also be somewhat odd,
  it is possible to make the request completely strict using a query
  flag '?strict', in which case, the server will only retrun results if
  strictly matching the request.