pgsql: Remove overspecification of precision of CURRENT_TIMESTAMP,

Log Message:
-----------
Remove overspecification of precision of CURRENT_TIMESTAMP, LOCALTIMESTAMP,
CURRENT_TIME, and LOCALTIME: now they just produce "timestamptz" not
"timestamptz(6)", etc.  This makes the behavior more consistent with our
choice to not assign a specific default precision to column datatypes.
It should also save a few cycles at runtime due to not having to invoke
the round-to-given-precision functions.
I also took the opportunity to translate CURRENT_TIMESTAMP into "now()"
instead of an invocation of the timestamptz input converter --- this should
save a few cycles too.

Modified Files:
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    pgsql/src/backend/parser:
        gram.y (r2.496 -> r2.497)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/parser/gram.y.diff?r1=2.496&r2=2.497)