postgres_fdw: Save foreign server OID in connection cache entry.

postgres_fdw: Save foreign server OID in connection cache entry.

The foreign server OID stored in the connection cache entry is used as
a lookup key to directly get the server name.

Previously since the connection cache entry did not have the server OID,
postgres_fdw had to get the server OID at first from user mapping before
getting the server name. So if the corresponding user mapping was dropped,
postgres_fdw could raise the error "cache lookup failed for user mapping"
while looking up user mapping and fail to get the server name even though
the server had not been dropped yet.

Author: Bharath Rupireddy
Reviewed-by: Fujii Masao
Discussion: https://postgr.es/m/CALj2ACVRZPUB7ZwqLn-6DY8C_UmPs6084gSpHA92YBv++1AJXA@mail.gmail.com