patch: streaming of bytea parameter values

This patch allows bytea parameters set via setBytes() /
setBinaryStream() to be streamed to the backend. With the patch applied,
the additional memory overhead for bytea parameters is small and
independent of how large the parameter data is.

Note that it doesn't touch the ResultSet path, so you'll still need a
pile of extra memory if you execute queries that return large bytea values.

It passes the driver's testcases against a 7.4 server, and doesn't show
any problems after brief testing with our bytea-using application. I've
had a report of strange things happening on the server->client path, but
don't have any details at the moment.

This needs testing under a real workload, and testing against a 7.3
server (the V2 protocol path is currently untested). Any volunteers?

-O