use configurable keys for keeping static connections #160

Job	Run time
CS-Fixer	13s
php-stan	23s
Test (7.4, --prefer-lowest, test_phpunit_legacy)	1m 25s
Test (7.4, 5.4.*, test_phpunit_legacy)	1m 29s
Test (8, test_phpunit_legacy)	1m 26s
Test (8.1, test)	54s
Test (8.2, 6.3.*, test)	1m 24s
Test (8.3, 6.4.*, test)	1m 3s
Test (8.2, 7.0.*, test)	1m 11s
Test (8.3, dev, test)	58s
Test (8.4, dev, test)	1m 0s
	11m 26s

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