In case an OrPostingIterator is below and AndPostingIterator, which is
the common case when using atleast two term, most of the documents will
be skipped.
Instead of skipping by repeatedly calling OrPostingIterator::next()
implement the skipTo method. This removes the overhead of looping over
the subsets for each next call.
When skipTo is implemented in the AndPostingIterator and
OrPostingIterator, the instruction count is significantly reduced,
e.g. a query for "the fox" goes down from 20M to 4.5M instructions
(query time 6ms vs 1.5ms), on a DB with 4.600 documents.
Depends on D28839