Support dynamic record as return type.

Support dynamic record as return type.

Now PL/Proxy allows function defined as RETURNS RECORD without
OUT parameters.  Such functions need actual result column
types specified with AS clause:

  SELECT * FROM func() AS (col1 type1, col2 type2);

PL/Proxy needs to detect the actual type on each call and pass
it to remote database.  Current implementation caches last
record type, drops it if type changes.

Original patch by Lei Yonghua, applied with some modifications.