Lets (not) break all the things. Was: [pgsql-advocacy] 9.6 -> 10.0

Moving over a conversation from the pgsql-advocacy mailing list.  In it
Simon (CC'd) raised the issue of potentially creating a backwards-compatibility
breaking release at some point in the future, to deal with things that
might have no other solution (my wording).

Relevant part of that thread there for reference:
 http://www.postgresql.org/message-id/CANP8+jLtk1NtaJyXc=hAqX=0k+ku4zfavgVBKfs+_sOr9hepNQ@mail.gmail.com

Simon included a short starter list of potentials which might be in
that category:
 * SQL compliant identifiers * Remove RULEs * Change recovery.conf * Change block headers * Retire template0, template1
*Optimise FSM * Add heap metapage * Alter tuple headers et al 

This still is better placed on -hackers though, so lets have the
conversation here to figure out if a "backwards compatibility breaking"
release really is needed or not.

Hopefully we can get it all done without giving users a reason to consider
switching. ;)

Regards and best wishes,

Justin Clift

--
"My grandfather once told me that there are two kinds of people: those
who work and those who take the credit. He told me to try to be in the
first group; there was less competition there."
- Indira Gandhi