Restructure/split up fetch.ts

[porcellus]

Current problems:

• There is a lot of repeated code between interceptors
• The current implementation is hard to change
• fetch.ts contains multiple classes with lots of separate responsibilites

We should:

• Make it clearer which classes/instances store the current state
• Make it easier to change by separating responsibilities into new classes/modules
• Make the possible states/state changes easier to see/follow.