Update extension lookup routines to use the syscache

Update extension lookup routines to use the syscache

The following routines are changed to use the syscache entries added for
pg_extension in 490f869d92e5:
- get_extension_oid()
- get_extension_name()
- get_extension_schema()

A catalog scan is costly and could easily lead to a noticeable
performance impact when called once or more per query, so this is going
to be helpful for developers for extension data lookups.

Author: Andrei Lepikhov
Reviewed-by: Jelte Fennema-Nio
Discussion: https://postgr.es/m/529295b2-6ba9-4dae-acd1-20a9c6fb8f9a@gmail.com