ReusableMessages not getting released when using entry/exit logging

## Description

I was getting some `ReusableMessageFactoryTest` failures in the release-2.x branch (without any modifications). It seemed that another test (TraceLoggingTest) was leaving an unreleased `ReusableParameterizedMessage` in the `ReusableMessageFactory` thread local. This only seems to be happening in my new dev environment (Azure VM) and is test order specific.

In the `DefaultFlowMessageFactory#newEntryMessage()`, entry ReusableMessages were being converted to immutable Messages and failing to release the original ReusableMessage.

In contrast, in `DefaultFlowMessageFactory#newExitMessage()`, ReusableMessages were being used directly (without being made immutable). I'm not sure if this was an oversight, but I decided to make both methods call into makeImmutable().

This issue would leave ReusableMessages reuse in a useless state after entry/exit logging was used by a thread.

## Configuration

**Version:** release-2.x
**Operating system:** Azure Linux VM
**JDK:** JDK8

## Logs
```
[ERROR] Failures:
[ERROR] ReusableMessageFactoryTest.testCreateEventOverwritesFields:81->assertReusableParameterizeMessage:58 reserved ==> expected: but was:
[ERROR] ReusableMessageFactoryTest.testCreateEventReturnsSameInstance:47 expected: but was:
[INFO]
[ERROR] Tests run: 683, Failures: 2, Errors: 0, Skipped: 2
```

## Reproduction

I'm not sure how to influence the order in which Maven runs tests. I added a test to TraceLoggingTest that shows that the thread local Message used is not being released.

```
// Ensure that ReusableParameterizedMessages are getting released correctly
@Test
public void testReusableMessageHandling() throws InstantiationException, IllegalAccessException {
assertSame(AbstractLogger.DEFAULT_MESSAGE_FACTORY_CLASS, ReusableMessageFactory.class,
"The default message factory has changed");

final MessageFactory factory = new ReusableMessageFactory();
final Message reusedMessage = factory.newMessage("test {}", null);
ReusableMessageFactory.release(reusedMessage);

traceEntry("Entry test {}", 0);
assertReusedMessage(reusedMessage, factory);

traceExit("Exit test {}", 0);
assertReusedMessage(reusedMessage, factory);
}

private void assertReusedMessage(Message reusableMessage, MessageFactory factory) {
final Message testMessage = factory.newMessage("test {}", 0);

assertSame(reusableMessage, testMessage);

ReusableMessageFactory.release(testMessage);
}
```