proposal, patch: allow multiple plpgsql plugins

Hello

We talked about enhancing a plpgsql plugin API to support more active
plugins.

I wrote a prototype based on function plpgsql_register_plugin instead
rendezvous variable.

There are two basic questions:

a) will we support rendezvous variable still?

b) will we support same API still - a reference on plugin_info in exec
state is a issue - described in patch.

without a) a d b) we will break a current plugins little bit more than is
usual - not terrible hard to fix it. But without a) and b) a
implementation can be significantly cleaner.

Comments, notes?

Regards

Pavel