ignore bws sync requests until initial sync is complete

[garyschulte]

PR description

Ensure we do not attempt to start BWS sessions if initial sync has not yet completed. This change prevents BWS from triggering via engine-api during:

• initial sync
• auto-heal
• resync worldstate

Fixed Issue(s)

[github-actions]

• I thought about documentation and added the doc-change-required label to this PR if updates are required.
• I thought about the changelog and included a changelog update if required.
• If my PR includes database changes (e.g. KeyValueSegmentIdentifier) I have thought about compatibility and performed forwards and backwards compatibility tests

[garyschulte]

noticed while testing:

{"@timestamp":"2024-01-24T01:46:36,934","level":"WARN","thread":"vert.x-worker-thread-0","class":"MergeCoordinator","message":"Internal error, backward sync completed but failed to import finalized block 0x96d1bee095e8c72586e2eeaf374d169e200865d3c4fced88abb0de949167b9b6","throwable":""}
{"@timestamp":"2024-01-24T01:46:48,500","level":"WARN","thread":"vert.x-worker-thread-0","class":"MergeCoordinator","message":"Internal error, backward sync completed but failed to import finalized block 0x96d1bee095e8c72586e2eeaf374d169e200865d3c4fced88abb0de949167b9b6","throwable":""}

[garyschulte]

backward sync completed but failed to import finalized block

Changed to return failed future rather than completed null future for BWS sync requests when BWS is not ready.