Extract out each DataStore to separate npm modules

[bhstahl]

Users should be able to choose their appropriate DataStore, and not deal with all stores. Right now all stores are included in this module, which causes issues such as #56 and #54.

This will serve as a progress issue, leading to the closing of #61.

• [ ] tus-datastore-abstract
• [ ] tus-datastore-filesystem
• [ ] tus-datastore-gcs

[kvz]

FWIF, for uppy.io we're considering something like https://lernajs.io/ (once we're 1.0), so that we get to keep all code in one repo, but still push out separate npm modules. advantages:

• easy to change structure across all modules while the project is still young, and propose those as a single pr/atomic unit
• one issue tracker
• share ci/testing boilerplate, build tools, etc without the need for duplication or (leaky) abstraction

[tkrugg]

looks like a dup for https://github.com/tus/tus-node-server/issues/61

[Murderlon]

Closing indeed as a duplicate of #61