Save flowNodeStore.xml entries in order

[jglick]

After jenkinsci/workflow-cps-plugin#807 by @dwnusbaum I started poking through large build records captured by support-core and was frustrated by the semi-random order of flow nodes.

Before fix:

java.lang.AssertionError: 

Expected: is <[2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20, 21, 22, 23, 24, 25, 26, 27, 28, 29, 30, 31, 32, 33, 34, 35, 36, 37, 38, 39, 40, 41, 42, 43]>
     but: was <[22, 23, 24, 25, 26, 27, 28, 29, 30, 31, 32, 10, 33, 11, 34, 12, 35, 13, 36, 14, 37, 15, 38, 16, 39, 17, 18, 19, 2, 3, 4, 5, 6, 7, 8, 9, 40, 41, 42, 20, 43, 21]>
	at org.hamcrest.MatcherAssert.assertThat(MatcherAssert.java:20)
	at org.hamcrest.MatcherAssert.assertThat(MatcherAssert.java:6)
	at org.jenkinsci.plugins.workflow.support.storage.BulkFlowNodeStorageTest.orderOfEntries(BulkFlowNodeStorageTest.java:58)