[WIP] Various improvements and bugfixes

• Implement in-process generation (cc #2)
• Backdated flakes (cc #1)
• Persist time for backwards detection (still wip, cc #3)
• Configurable worker strategy (still wip, cc #4)
• Follow system clock by using os:system_time instead of

erlang:system_time (fixes pleroma/pleroma#1681)

• Refuse to generate flakes when the clock is not advancing and the

sequence overflows

• Split up building from Worker to a dedicated module Builder