Fix edge case where state sync is not triggered (#5635)

This PR addresses an issue where state sync may fail to start if the
conditions required for its initiation are not met when a finalized
block notification is received. `pending_state_sync_attempt` is
introduced to trigger the state sync later when the conditions are
satisfied.

This issue was spotted when I worked on #5406, specifically,
`queue_blocks` was not empty when the finalized block notification was
received, and then the state sync was stalled. cc @dmitry-markin



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Co-authored-by: Dmitry Markin <dmitry@markin.tech>
Co-authored-by: Bastian Köcher <git@kchr.de>