Statement timeout in pg_rewind

Hi,

It is quite common to set a global statement_timeout to a few seconds
(or minutes) in postgresql.conf in order to avoid hitting a production
server with slow/bad queries.
This value is applied to all connections in the system unless it is
redefined per database, user, or explicitly changed in the connection.
Pg_rewind runs quite a few queries on the primary and if
statement_timeout hits one of them it breaks the whole process. I
can't tell for sure if this is an unrecoverable error or not and maybe
the second run of pg_rewind would be able to finish the process. Even
in case if retry is possible it makes it hard to use it for reliable
automation.

There are a few workarounds to this problem:
1. ALTER DATABASE postgres SET statement_timeout = 0;
2. ALTER rewind_username SET statement_timeout = 0;
3. Run export PGOPTIONS="-c statement_timeout=0" before calling pg_rwind.

All of them have certain pros and cons. The third approach works good
for automation, but IMHO we should simply fix pg_rewind itself and SET
statement_timeout after establishing a connection, so everybody will
benefit from it.

Patch attached.

Regards,
--
Alexander Kukushkin