(1) do not use the nsloc index if the context node set is small (<logN) (2) small perf improvement in log2 function
(1) do not use the nsloc index if the context node set is small (<logN)
(2) small perf improvement in log2 function

get_security_price should is faster now => 15msec on 1GB
but (1) will improve performance across the board

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