[WIP] Various improvements and bugfixes

• Implement in-process generation (fixes #2)
• Backdated flakes (fixes #1)
• Persist time for backwards detection (fixes #3)
• Configurable worker strategy (fixes #4)
• Follow system clock by using os:system_time instead of erlang:system_time (fixes pleroma/pleroma#1681 (closed))
• Refuse to generate flakes when the clock is not advancing and the sequence overflows
• Split up building from Worker to a dedicated module Builder
• Docs improvements
• tests for local & backdate

BTW I think that once this MR is merged we should release a 1.0, as it's now feature complete and the API won't change.