Peer doesn't fetch private data when BTL lower than 10

Peer has a config option named "btlPullMargin" which default value is 10. In config comment describes, it use a buffer to prevent peer from trying to pull private data from peers that is soon to be purged in next N blocks.

If blockToLive in collection_config.json lower than 10, peer that newly join channel or newly scaled won't fetch any private data from other peers.