COPY performance improvements. Avoid calling CopyGetData for each input

character, tighten the inner loops of CopyReadLine and CopyReadAttribute,
arrange to parse out all the attributes of a line in just one call instead
of one CopyReadAttribute call per attribute, be smarter about which client
encodings require slow pg_encoding_mblen() loops.  Also, clean up the
mishmash of static variables and overly-long parameter lists in favor of
passing around a single CopyState struct containing all the state data.
Original patch by Alon Goldshuv, reworked by Tom Lane.