Send least-replicated symbol to new nodes in log append

[btmorr]

When adding a node to the cluster or writing logs to a non-voting node, for each log entry the leader should send whichever symbol is least-frequently replicated across the cluster (tracked per #96). The leader can read the needed symbol from whichever member(s) of the cluster currently have a copy, or if none are available the leader should read enough symbols to reconstruct the original data and re-generate the symbol.