rw_redis_fdw: SQL Errors when statement is within a function

Hi,

I have faced an incident which, according to the developer, "The
problem seems to be that pgsql switches from a CONST sub-expression
into a FUNCEXPR after a few goes, ..."

Please have a look at the following two links which describe the problem:

https://github.com/nahanni/rw_redis_fdw/issues/13
https://github.com/nahanni/rw_redis_fdw/issues/14

The first link (13) has to do with PG10.x version.
The second one (14) has to do with PG11 version in which the developer
traces the "problem".

Please check and comment.

Tia