Introduce ClassMetadataFactoryInterface to allow switching implementations #957

Job	Run time
PHPUnit (8.0, 6.0, stable, server, highest)	1m 30s
PHPUnit (8.0, 5.0, stable, server, highest)	1m 9s
PHPUnit (8.0, 4.4, stable, server, highest)	1m 12s
PHPUnit (8.1, 6.0, stable, server, highest)	1m 43s
PHPUnit (8.1, 5.0, stable, server, highest)	1m 17s
PHPUnit (8.1, 4.4, stable, server, highest)	1m 16s
PHPUnit (8.2, 6.0, stable, server, highest)	1m 35s
PHPUnit (8.2, 5.0, stable, server, highest)	1m 12s
PHPUnit (8.2, 4.4, stable, server, highest)	1m 16s
PHPUnit (lowest, 8.0, 4.4, 1.11.0, server)	2m 30s
PHPUnit (sharded_cluster, 8.2, 4.4, stable, highest)	2m 30s
	17m 10s

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