Upstream commit 43a57cf3, which significantly changes the API for the
HashBitmap (TIDBitmap in Postgres), is about to hit in an upcoming
merge. This patch is a joint effort by myself, Max Yang, Xiaoran Wang,
Heikki Linnakangas, and Daniel Gustafsson to reduce our diff against
upstream and support the incoming API changes with our GPDB-specific
customizations.

The primary goal of this patch is to support concurrent iterations over
a single StreamBitmap or TIDBitmap. GPDB has made significant changes to
allow either one of those bitmap types to be iterated over without the
caller necessarily needing to know which is which, and we've kept that
property here.

Here is the general list of changes:

- Cherry-pick the following commit from upstream:

  commit 43a57cf3
  Author: Tom Lane <tgl@sss.pgh.pa.us>
  Date:   Sat Jan 10 21:08:36 2009 +0000

    Revise the TIDBitmap API to support multiple concurrent iterations over a
    bitmap.  This is extracted from Greg Stark's posix_fadvise patch; it seems
    worth committing separately, since it's potentially useful independently of
    posix_fadvise.

- Revert as much as possible of the TIDBitmap API to the upstream
  version, to avoid unnecessary merge hazards in the future.

- Add a tbm_generic_ version of the API to differentiate upstream's
  TIDBitmap-only API from ours. Both StreamBitmap and TIDBitmap can be
  passed to this version of the API.

- Update each section of code to use the new generic API.

- Fix up some memory management issues in bitmap.c that are now
  exacerbated by our changes.