matrix-synapse: limit parallelism to 4 cores to be improve stability, formatting

On high core machines those easily can fail similar to:

matrix-synapse> tests.storage.test_room_search.MessageSearchTest.test_postgres_web_search_for_phrase
matrix-synapse> ===============================================================================
matrix-synapse> [ERROR]
matrix-synapse> Traceback (most recent call last):
matrix-synapse>   File "/build/source/tests/unittest.py", line 122, in new
matrix-synapse>     return code(orig, *args, **kwargs)
matrix-synapse>   File "/build/source/tests/unittest.py", line 216, in setUp
matrix-synapse>     return orig()
matrix-synapse>   File "/build/source/tests/unittest.py", line 338, in setUp
matrix-synapse>     self.hs = self.make_homeserver(self.reactor, self.clock)
matrix-synapse>   File "/build/source/tests/app/test_openid_listener.py", line 34, in make_homeserver
matrix-synapse>     hs = self.setup_test_homeserver(
matrix-synapse>   File "/build/source/tests/unittest.py", line 606, in setup_test_homeserver
matrix-synapse>     hs = setup_test_homeserver(self.addCleanup, **kwargs)
matrix-synapse>   File "/build/source/tests/server.py", line 921, in setup_test_homeserver
matrix-synapse>     prepare_database(
matrix-synapse>   File "/nix/store/2cc0p5apn2yg2fr5ii9mvb7fcwd74y26-matrix-synapse-1.86.0/lib/python3.10/site-packages/synapse/storage/prepare_database.py", line 155, in prepare_database
matrix-synapse>     raise UpgradeDatabaseException(EMPTY_DATABASE_ON_WORKER_ERROR)
matrix-synapse> synapse.storage.prepare_database.UpgradeDatabaseException: Uninitialised database: run the main synapse process to prepare the database schema before starting worker processes.
matrix-synapse>

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