Mark btree_gist functions as PARALLEL SAFE

Hi,

On the back of this thread[1] over at pgsql-general, I've attached a patch that marks the functions in btree_gist as
PARALLELSAFE.
 
This is primarily to allow parallel plans to be considered when btree_gist's <-> operator is used in any context; for
examplein an expression
 
that will be evaluated at execution time, or in a functional column in btree or gist indexes.

In the latter example, despite the functions already being marked IMMUTABLE, attempts to retrieve precomputed values
froma functional index during an index scan or index-only scan still require the function to be marked PARALLEL SAFE to
preventdropping down to a serial plan.
 

It requires btree_gist's version to be bumped to 1.6.

In line with this commit[2], and for the same reasons, all functions defined by btree_gist are being marked as safe:

---

"... Note that some of the markings added by this commit don't have any
effect; for example, gseg_picksplit() isn't likely to be mentioned
explicitly in a query and therefore it's parallel-safety marking will
never be consulted.  But this commit just marks everything for
consistency: if it were somehow used in a query, that would be fine as
far as parallel query is concerned, since it does not consult any
backend-private state, attempt to write data, etc."

---

I haven't added any more tests, but neither did I find any added with the above commit.

"CREATE EXTENSION btree_gist" runs successfully, as does "make check-world".

This is the first patch I've submitted, so if I've omitted something then please let me know.
Thanks for your time,
Steven.


[1]
https://www.postgresql.org/message-id/DB7PR09MB2537E18FF90C1C1BBF49D628FD830%40DB7PR09MB2537.eurprd09.prod.outlook.com
[2] https://github.com/postgres/postgres/commit/2910fc8239fa501b662c5459d7ba16a4bc35e7e8


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