Introduce thread safety to SAML schema read #394

Job	Run time
Unit test (ubuntu-20.04, 2.1)	54s
Unit test (ubuntu-20.04, 2.2)	1m 3s
Unit test (ubuntu-20.04, 2.3)	1m 6s
Unit test (ubuntu-20.04, 2.4)	1m 1s
Unit test (ubuntu-20.04, 2.5)	28s
Unit test (ubuntu-20.04, 2.6)	29s
Unit test (ubuntu-20.04, 2.7)	28s
Unit test (ubuntu-20.04, 3)	16s
Unit test (ubuntu-20.04, 3.1)	17s
Unit test (ubuntu-20.04, 3.2)	18s
Unit test (ubuntu-20.04, 3.3)	16s
Unit test (ubuntu-20.04, jruby-9.1)	55s
Unit test (ubuntu-20.04, jruby-9.2)	59s
Unit test (ubuntu-20.04, jruby-9.3)	49s
Unit test (ubuntu-20.04, jruby-9.4)	52s
Unit test (ubuntu-20.04, truffleruby)	1m 51s
Unit test (macos-latest, 2.6)	38s
Unit test (macos-latest, 2.7)	41s
Unit test (macos-latest, 3)	21s
Unit test (macos-latest, 3.1)	20s
Unit test (macos-latest, 3.2)	19s
Unit test (macos-latest, 3.3)	24s
Unit test (macos-latest, jruby-9.3)	36s
Unit test (macos-latest, jruby-9.4)	38s
Unit test (macos-latest, truffleruby)	1m 31s
Unit test (windows-latest, 2.2)	1m 38s
Unit test (windows-latest, 2.3)	1m 36s
Unit test (windows-latest, 2.4)	1m 33s
Unit test (windows-latest, 2.5)	3m 0s
Unit test (windows-latest, 2.6)	2m 10s
Unit test (windows-latest, 2.7)	2m 47s
Unit test (windows-latest, 3)	1m 23s
Unit test (windows-latest, 3.1)	1m 30s
Unit test (windows-latest, 3.2)	1m 15s
Unit test (windows-latest, 3.3)	2m 52s
finish	2s
	37m 16s

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