pgsql: Make plperl safe against functions that are redefined while runn

Make plperl safe against functions that are redefined while running.

validate_plperl_function() supposed that it could free an old
plperl_proc_desc struct immediately upon detecting that it was stale.
However, if a plperl function is called recursively, this could result
in deleting the struct out from under an outer invocation, leading to
misbehavior or crashes.  Add a simple reference-count mechanism to
ensure that such structs are freed only when the last reference goes
away.

Per investigation of bug #7516 from Marko Tiikkaja.  I am not certain
that this error explains his report, because he says he didn't have
any recursive calls --- but it's hard to see how else it could have
crashed right there.  In any case, this definitely fixes some problems
in the area.

Back-patch to all active branches.

Branch
------
REL8_4_STABLE

Details
-------
http://git.postgresql.org/pg/commitdiff/dc447e1515941c8edc51fabcd318f7b1311dd1e8

Modified Files
--------------
src/pl/plperl/expected/plperl.out |   18 +++++++
src/pl/plperl/plperl.c            |   95 +++++++++++++++++++++++++-----------
src/pl/plperl/sql/plperl.sql      |   10 ++++
3 files changed, 94 insertions(+), 29 deletions(-)