Fix bugs around handling of params passed to the datanodes.

Fix bugs around handling of params passed to the datanodes.

This fixes problems reported in plpgsql function calls since
fast-query-shipping was added. But fixing the problem also highlighted other
bugs in the protocol handling. For example, we would set a datanode connection
state to be IDLE upon receiving a CommandComplete. But for simple query
protocol, we should really be waiting for ReadyForQuery before changing the
state. This patch has related fixes.

plpgsql test case's expected output is also fixed now that the underlying bug
has been take care of