s/idx/heapIndex/g and remove its unnecessary cache
s/idx/heapIndex/g and remove its unnecessary cache

benchmark                              old ns/op     new ns/op     delta
Benchmark_10values_windowsize1         512           486           -5.08%
Benchmark_100values_windowsize10       8893          8961          +0.76%
Benchmark_10Kvalues_windowsize100      1711388       1703878       -0.44%
Benchmark_10Kvalues_windowsize1000     2393338       2440797       +1.98%

JaderDias/movingmedian