Hot Standby and query cancel

We've been chewing around query cancel on Hot Standby and I think things
have got fairly confusing, hence a new thread.

I enclose a patch that includes all the things that we all agree on so
far, in my understanding

* Recovery conflict processing uses SIGUSR1 rather than shmem per Tom,
while holding ProcArrayLock per Andres

* CONFLICT_MODE_ERROR throws ERROR when in a transaction, not idle and
not in subtransaction, otherwise becomes CONFLICT_MODE_FATAL per Tom and
other discussion

* Recovery abort message has additional detail, per Heikki

It doesn't include anything still under discussion, though is intended
as a base upon which further patches can progress independently.

* Infrastructure for supercancel, by Joachim Wieland

* Any of the many further ideas by Andres Freund

Please review this so we can move onto taking other issues one by one.
This is also a base for other HS work I need to complete.

I am still testing patch, so should be confident to commit tomorrow
barring issues.

--
 Simon Riggs           www.2ndQuadrant.com