Remove mandatory API key checks

[hydra-yse]

Built on top of #637
This PR:

• Makes the real-time sync service optional (depending on the presence of sync_service_url) as follows:
  • Set and equal to BREEZ_SYNC_SERVICE_URL: Check for API key
  • Set and different from BREEZ_SYNC_SERVICE_URL: Use without API key
  • Not set: Disable the sync service
• Conditionally calls rt-sync commit methods to avoid cluttering of the db when the sync service is not active
• Skips the initial validate_api_key call unless it is set in the config

This should complete all the requirements for #554, except for the swapper proxy changes & fiat api.

[roeierez]

@hydra-yse this needs rebase on top of the base branch.