fix(eth/downloader): fix sync fall behind because peer was reported timeout

[gzliudan]

Proposed changes

The QoS constants were shrunk relative to upstream go-ethereum, which pinned requestTTL() to a hard 5 second ceiling: ttlLimit was 5s while ttlScaling (2) multiplied by rttMaxEstimate (5s) already exceeded it, so neither the measured RTT nor the confidence factor could ever influence the timeout.

Any peer that did not answer a single header request within 5 seconds was therefore reported as errTimeout and dropped. On a busy network this made a node that had fallen behind unable to recover: every sync round died in fetchHeight, the node dropped healthy peers faster than it could replace them, and it only advanced through propagated blocks, which cannot close a multi-thousand block gap.

Restore rttMaxEstimate, ttlScaling and ttlLimit to the upstream values so the timeout scales with the observed round-trip time again, and add a test covering requestTTL().

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