[Feature][Perf] Trigger `try_advancing_block_synchronization` on events

[vicsn]

🚀 Feature

We should trigger try_advancing_block_synchronization, not at a fixed interval but on specific events. It should just be the following

New block locators were received
All outstanding block requests were processed
A peer disconnects (can affect outstanding block requests)

We can still keep the timer as a failsafe.

We might need to avoid triggering sync while it is still in progress.