astrid: optimise and tidy up synchronisation points

[taratorio]

Before this PR we called heimdall.Synchronize as part of heimdall.CheckpointsFromBlock and heimdall.MilestonesFromBlock. The previous implementation of Synchronize was waiting on all scrappers to be synchronised.

This is inefficient because heimdall.CheckpointsFromBlock needs only the checkpoints scrapper to be synchronised. For the initial sync we first only need to wait for the checkpoints to be downloaded and then we can start downloading blocks from devp2p. While we are doing that we can let the spans and milestones be scrapped in the background. Note this is based on the fact that fetching checkpoints has been optimised by doing bulk fetching and finishes in seconds, while fetching Spans has not yet been optimised and for bor-mainnet can take a long time.

Changes in the PR:

• splits Synchronize into 3 more fine grained SynchronizeCheckpoints, SynchronizeMilestones and SynchronizeSpans calls which are invoked by the Sync algorithm at the right time
• Optimises SynchronizeSpans to check if it already has the corresponding span for the given block number before blocking
• Moves synchronisation point for Spans and State Sync Events in Sync.commitExecution just before we call ExecutionEngine.UpdateForkChoice to make it clearer what data is necessary to be sync-ed before calling Execution
• Changes EventNotifier and Synchronize funcs to return err if ctx is cancelled or other errors have happened
• Input consistency between the heimdallSynchronizer and bridgeSynchronizer - use blockNum instead of *type.Header
• Interface tidy ups