pgsql: COPY performance improvements.

Log Message:
-----------
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.

Modified Files:
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    pgsql/src/backend/commands:
        copy.c (r1.247 -> r1.248)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/commands/copy.c.diff?r1=1.247&r2=1.248)