Speed up bitmap creation by loading reverse index in parallel
to reading bitmap from storage. Latency changes from
(time_to_read_bitmap + time_to_load_reverse_index) to
max(time_to_read_bitmap, time_to_load_reverse_index).
Add new option to DfsReaderOptions to control parallel reverse index
loading. Static cached thread pool is added to PackBitmapIndexV1 for
reverse index loading, and when not in use consumes minimal resources.
Signed-off-by: Alina Djamankulova <adjama@google.com>
Change-Id: Ia37a1d739631d053e8bddb925ac8b0b81d22379e
4e915f9568