lookup info in the relcache for index access method support functions.
This makes a huge difference for dynamically loaded support functions,
and should save a few cycles even for built-in ones. Also tweak dfmgr.c
so that load_external_function is called only once, not twice, when
doing fmgr_info for a dynamically loaded function. All per performance
gripe from Teodor Sigaev, 5-Oct-01.
in plpgsql: they fail for datatypes that have old-style I/O functions
due to caching FmgrInfo structs with wrong fn_mcxt lifetime.
Although the plpython fix seems straightforward, I can't check it here
since I don't have Python installed --- would someone check it?
