btree: Support parallel index scans.

This isn't exposed to the optimizer or the executor yet; we'll add
support for those things in a separate patch.  But this puts the
basic mechanism in place: several processes can attach to a parallel
btree index scan, and each one will get a subset of the tuples that
would have been produced by a non-parallel scan.  Each index page
becomes the responsibility of a single worker, which then returns
all of the TIDs on that page.

Rahila Syed, Amit Kapila, Robert Haas, reviewed and tested by
Anastasia Lubennikova, Tushar Ahuja, and Haribabu Kommi.