Fix syncing websocket subscriptions

[tgmichel]

This PR adds missing information to the syncing subscriptions and fixes the behaviour to match what is documented in the web3js library.

Changes:

• Serialize response either as a boolean (on syncing false) or an object describing the syncing progress (on syncing true).
• Return syncing status on subscription instead of always depending on import notifications.
• If the node was major syncing on subscription, rely on import notification to emit an additional notification when the node is not major syncing anymore.