Consider adding watcher_syncing to /status.get

[unnawut]

Originated from omgnetwork/web-wallet#67.

When a user wants to check whether the watcher is in sync, the user has to compare /status.get's last_seen_eth_block_number against all services_synced_heights and if the gaps are all below a certain threshold, it's considered in sync.

This would be more convenient if the watchers expose their sync status above with e.g. watcher_syncing: true/false