pgsql: 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: /messages/by-id/529295b2-6ba9-4dae-acd1-20a9c6fb8f9a@gmail.com

Branch
------
master

Details
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https://git.postgresql.org/pg/commitdiff/d8df7ac5c04cd17bf13bd3123dcfcaf8007c6280

Modified Files
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src/backend/commands/extension.c | 82 +++++++---------------------------------
1 file changed, 13 insertions(+), 69 deletions(-)