Use appendStringInfoString and appendPQExpBufferStr where possible

Use appendStringInfoString and appendPQExpBufferStr where possible

This changes various places where appendPQExpBuffer was used in places
where it was possible to use appendPQExpBufferStr, and likewise for
appendStringInfo and appendStringInfoString.  This is really just a
stylistic improvement, but there are also small performance gains to be
had from doing this.

Discussion: http://postgr.es/m/CAKJS1f9P=M-3ULmPvr8iCno8yvfDViHibJjpriHU8+SXUgeZ=w@mail.gmail.com