Reviewing freeze map code

Hi,

The freeze map changes, besides being very important, seem to be one of
the patches with a high risk profile in 9.6.  Robert had asked whether
I'd take a look.  I thought it'd be a good idea to review that while
running tests for
http://www.postgresql.org/message-id/CAMkU=1w85Dqt766AUrCnyqCXfZ+rsk1witAc_=v5+Pce93Sftw@mail.gmail.com

For starters, I'm just going through the commits. It seems the relevant
pieces are:

a892234 Change the format of the VM fork to add a second bit per page.
77a1d1e Department of second thoughts: remove PD_ALL_FROZEN.
fd31cd2 Don't vacuum all-frozen pages.
7087166 pg_upgrade: Convert old visibility map format to new format.
ba0a198 Add pg_visibility contrib module.

did I miss anything important?

Greetings,

Andres Freund