spv: Move getheaders stage to global syncer instead of per-peer #2300
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This moves the fetch headers and cfilters stage from the per-peer startup function to the main syncer startup function.
This avoids wasting duplicate effort for every peer. In the future, this will allow using multiple peers to fetch headers and cfilters from, as well as allowing asynchronous fetching.
This change significantly reduces the amount of resources (cpu, ram, goroutines) used during the initial sync at the cost of increasing the sync time. The sync time will be reduced in the future as well, after parallelizing the headers/cfilters/chainswitch stages and after batched cfilter fetching is implemented and deployed.
Part of #2289