After discussing this patch privately with Andres I created a new
version of this patch.
The main changes are:
1. Build on top of a refactor to addrinfo handling I had done for
another patch of mine (libpq load balancing). This allows creation of
a fake addrinfo list, which made it possible to remove lots of special
cases for cancel requests from PQconnectPoll
2. Move -2 return value of pqReadData to a separate commit.
3. Move usage of new cancel APIs to a separate commit.
4. Move most of the logic that's specific to cancel requests to cancel
related functions, e.g. PQcancelPoll does more than simply forwarding
to PQconnectPoll now.
5. Copy over the connhost data from the original connection, instead
of assuming that it will be rebuilt identically in the cancel
connection. The main reason for this is that when/if the loadbalancing
patch gets merged, then it won't necessarily be rebuilt identically
anymore.