v0.18.0: Respect payout period, SSL fix, network and logging improvements

Major improvements since v0.17.0:

• Coinbase transactions are now only generated at each payout period instead of each block;
• The /validators endpoint will now return an empty result instead of an exception when called
  with a future block that doesn't have validators;
• The stats endpoint (/metrics) default timeout is not 70s, up from 10s, to cover Prometheus' default (60s);
• File logging will now flush every line after writing it, reducing writing delays;
• OpenSSL bindings have been updated to a more recent version;