spv: Fetch cfilters from multiple remote peers

[matheusd]

This modifies the cfilter fetching stage of the initial sync process to request cfilters from multiple peers instead of a single one.

This spreads the load across multiple peers, making the initial sync process (on average) slightly faster due to lower remote peer resource usage.

[jrick]

the watchdog commit looks like a prime candidate for context.WithoutTimeoutCause, where we define our special error, and check for that, instead of checking that the watchdog context errored but the outer context did not.

unfortunately, that would require go 1.21 as a minimum, so we can't use that yet.