Handle race condition between File.length() and FileChannel.size()

[epickrram]

Observed in the wild:

java.lang.IndexOutOfBoundsException: index=0 length=4 capacity=0
        at org.agrona.concurrent.UnsafeBuffer.boundsCheck0(UnsafeBuffer.java:1340)
        at org.agrona.concurrent.UnsafeBuffer.getIntVolatile(UnsafeBuffer.java:523)
        at org.agrona.MarkFile.mapExistingMarkFile(MarkFile.java:382)
        at org.agrona.MarkFile.<init>(MarkFile.java:178)
        at io.aeron.cluster.ClusterMarkFile.<init>(ClusterMarkFile.java:117)

This seems to be caused by File.length() returning a value greater than zero, passing this test:

while (!markFile.exists() || markFile.length() <= 0)

but then causing a zero-length buffer to be mapped here:

mappedByteBuffer = channel.map(READ_WRITE, 0, channel.size());

This PR adds a wait for FileChannel.size() to return at least 4, the minimum required to read the version number. I've also added a test to ensure that a more meaningful error message would be thrown in the event of a timeout.