We select a peer to ask a block randomly. Peer's send channel has
capacity 10. If we need too many blocks we limit number of blocks to asks as a number of peers
* 10, which means that there is some probability (pretty high) that we
will overflow send buffer capacity.
This fix freezes a peer list (which gives also some performance boost)
and create a cycle iteraror to equally distribute requests among the
peers.
There is a risk that a peer may be disconnected while we are sending a
request to the chanel, but stricltly speaking it was possible in the old
code too, perhaps with a lower probability.
Fixes#1748
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466ce986a6