pgsql: test_dsa: Avoid leaking LWLock tranches.

test_dsa: Avoid leaking LWLock tranches.

Since this is a test module, leaking a couple of LWLock tranches is
fine, but we want to discourage that pattern in third-party code.
This commit teaches the module to create only one tranche and to
store its ID in shared memory for use by other backends.

Reported-by: Alexander Lakhin <exclusion(at)gmail(dot)com>
Reviewed-by: Sami Imseih <samimseih(at)gmail(dot)com>
Discussion: https://postgr.es/m/dd36d384-55df-4fc2-825c-5bc56c950fa9%40gmail.com

Branch
------
master

Details
-------
https://git.postgresql.org/pg/commitdiff/c5c74282f2ea222beb7e07fc91b7afd7a06020c2

Modified Files
--------------
src/test/modules/test_dsa/test_dsa.c | 27 +++++++++++++++++++--------
1 file changed, 19 insertions(+), 8 deletions(-)