Optimise RA server init

[kjnilsson]

Dont write index 0 synchronously and don't fsync the config file to reduce blocking in gen_state:init function.

[mkuratczyk]

I see ~50% improvement compared to main:

• 10k QQs, 60s before this change, 30s after
• 50k QQs, 302s before, 152s after

For reference, 50k CQs takes 20s, 50k Streams takes 60s on the same machine

[kjnilsson]

Closing in favour of https://github.com/rabbitmq/ra/pull/333