Put back encoding-conversion step in processing of incoming queries;

I had inadvertently omitted it while rearranging things to support
length-counted incoming messages.  Also, change the parser's API back
to accepting a 'char *' query string instead of 'StringInfo', as the
latter wasn't buying us anything except overhead.  (I think when I put
it in I had some notion of making the parser API 8-bit-clean, but
seeing that flex depends on null-terminated input, that's not really
ever gonna happen.)