Remove overspecification of precision of CURRENT_TIMESTAMP, LOCALTIMESTAMP,

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.