Update for OTP20/rebar3.

[cmeiklejohn]

• gen_fsm_compat changes
• Fix seeds to use proper format
• Add nowarn_export_all on modules that explicitly export all.
• Switch to os:timestamp where possible.

[jadeallenx]

Damn, brings back good memories. 😢

[martinsumner]

@martincox

We have conflicts with this now, as you have made some direct commits. We need to work out how to resolve.

[martincox]

My bad. :man_facepalming: