Tests in DefaultMetadataTabletMapIT should be redesigned

[dkropachev]

After recent changes in scylla it start triggering tablets migration early on.
Some of the tests in DefaultMetadataTabletMapIT (in particular should_receive_each_tablet_exactly_once) assume that once driver got a tablet record from server it will never get it again, which is not true now.
As result these tests fail time to time.

Example of such failure:

Error:  Tests run: 2, Failures: 1, Errors: 0, Skipped: 0, Time elapsed: 20.136 s <<< FAILURE! - in com.datastax.oss.driver.core.metadata.DefaultMetadataTabletMapIT
Error:  com.datastax.oss.driver.core.metadata.DefaultMetadataTabletMapIT.should_receive_each_tablet_exactly_once  Time elapsed: 0.825 s  <<< FAILURE!
java.lang.AssertionError: expected:<32> but was:<33>
	at org.junit.Assert.fail(Assert.java:89)
	at org.junit.Assert.failNotEquals(Assert.java:835)
	at org.junit.Assert.assertEquals(Assert.java:647)
	at org.junit.Assert.assertEquals(Assert.java:633)
	at com.datastax.oss.driver.core.metadata.DefaultMetadataTabletMapIT.should_receive_each_tablet_exactly_once(DefaultMetadataTabletMapIT.java:321)