Request for review of new redis-fdw module

Hi

I've implemented a completely new Redis FDW module which has little to do
with github.com/pg-redis-fdw/redis_fdw; although I did take some
inspiration from in on how the tables were to be designed but most I got
from looking at the oracle-fdw.

My redis-fdw implementation supports read and write to the Redis backend,
so you can do insert, update, and delete. e.g. you can define a hash table
as:

table rhash (
key text,
field text,
value test,
expiry int
) server redserver options(tabletype 'hash');

and do:
select * from rhash where key = 'foo' and field = 'bar';
update rhash set value = 'bahbah' where key = 'foo' and field = 'bar';
delete from rhash where key = 'foo' and field = 'bar';

I need someone experienced with postgresql's internals for FDWs to review
my code and let me know if I've done something wrong or where I can
optimize it a little more. Once it has been reviewed, I'll post it up and
announce it on my github account for public consumption. But for now I want
to make sure that the code is correct.

There are some functions (Explain* and costs) which I know I haven't
implemented properly so assistance with that is also appreciated.

Please email me if you can assist with reviewing.

thanks
Leon