Extend compatibility of PostgreSQL::Test::Cluster

PFA a patch to extend the compatibility of PostgreSQL::Test::Cluster to
all live branches. It does this by introducing a couple of subclasses
which override a few things. The required class is automatically
detected and used, so users don't need to specify a subclass. Although
this is my work it draws some inspiration from work by Jehan-Guillaume
de Rorthais. The aim here is to provide minimal disruption to the
mainline code, and also to have very small override subroutines.

My hope is to take this further, down to 9.2, which we recently decided
to give limited build support to. However I think the present patch is a
good stake to put into the ground.

cheers

andrew

--
Andrew Dunstan
EDB: https://www.enterprisedb.com