We try to predict the next headers the interface will ask for,
and request them ahead of time, to be kept in the headers_cache.
This saves network latency/round-trips, for a bit more memory usage
and in some cases for more bandwidth.
Note that due to PaddedRSTransport.WAIT_FOR_BUFFER_GROWTH_SECONDS,
latency saved here can be longer than "real" network latency.
This speeds up
- binary search greatly,
- backwards search to a small degree
(although not that much as its algorithm should be changed a bit to make it cache-friendly)
- catch-up greatly, if it's <10 blocks behind
What remains is to speed up catch-up in case we are behind by many thousands of block.
That behaviour is left unchanged here. The issue there is that we request chunks sequentially.
So e.g. 1 chunk (2016 blocks) per 1 second.
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