Make MMAP_SIZE_THRESHOLD in OffHeapSingleTreeBuilder builder is configurable

[chrajeshbabu]

Currently when the buffer size required for sort and aggregate the records require for star tree building above 500mb using MMAP based files used which is making star tree building slower compared to direct memory usage when the number of docs and dimensions are higher. Would be better to make it configurable so based on the available memory and number of segments to build the startree.

  // If the temporary buffer needed is larger than 500M, use MMAP, otherwise use DIRECT
  private static final long MMAP_SIZE_THRESHOLD = 500_000_000;