Speed up plpgsql trigger startup by introducing "promises".

Over the years we've accreted quite a few special variables that are
predefined in plpgsql trigger functions.  The cost of initializing these
variables to their defined values turns out to be a significant part of
the runtime of simple triggers; but, undoubtedly, most real-world triggers
never examine the values of most of these variables.

To improve matters, invent the notion of a variable that has a "promise"
attached to it, specifying which of the predetermined values should be
assigned to the variable if anything ever reads it.  This eliminates all
the unneeded startup overhead, in return for a small penalty on accesses
to these variables.

Tom Lane, reviewed by Pavel Stehule

Discussion: https://postgr.es/m/11986.1514407...@sss.pgh.pa.us

Branch
------
master

Details
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https://git.postgresql.org/pg/commitdiff/fd333bc763ea104f2a2c10c6b0061c996d4a2f5a

Modified Files
--------------
src/pl/plpgsql/src/pl_comp.c  |  50 +++++--
src/pl/plpgsql/src/pl_exec.c  | 334 +++++++++++++++++++++++++++---------------
src/pl/plpgsql/src/pl_funcs.c |   5 +
src/pl/plpgsql/src/pl_gram.y  |   3 +
src/pl/plpgsql/src/plpgsql.h  |  56 +++++--
5 files changed, 306 insertions(+), 142 deletions(-)

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