JIRA: https://jira.spring.io/browse/INT-4237
It was possible to flush (close) the file while a write was in process; more likely when
`flushWhenIdle` is false.
This probably would not occur in the real world, just tests with short flush intervals,
but certainly possible.
There is already a lock used to prevent concurrent writes while appending; use the same
lock when flushing.
Conflicts:
spring-integration-file/src/main/java/org/springframework/integration/file/FileWritingMessageHandler.java
spring-integration-file/src/test/java/org/springframework/integration/file/FileWritingMessageHandlerTests.java
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