BOSS: add caching

[jstriebel]

This PR:

• makes the BOSS backend cuboid aware, and requests chunks in those sizes
• uses caching for data chunks
• adds some refactorings:
  • avoid searching lists in BOSS and neuroglancer backend, using dicts instead
  • moves _chunks and _cutout to Backend baseclass (maybe undoing this, if we don't align to cuboids in BOSS)
  • some minor stuff

~~Using the cuboids delayes the arrival of the first chunks heavily, which gives a bad UX I would rather not use cuboid chunks here, and instead cache the chunks in the size of the incoming requests.~~

fixes #61

[jstriebel]

I removed the cuboid alignment from this PR, as discussed with @fm3 . Feel free to review :eyes:

[jstriebel]

@fm3 Thanks for the feedback! I just pushed a commit which addresses it, does this look fine?