Fix intermittent PreLeaderScenarioTest failure

Fix intermittent PreLeaderScenarioTest failure

testUnComittedEntryOnLeaderChange(org.opendaylight.controller.cluster.raft.PreLeaderScenarioTest)  Time elapsed: 5.207 sec  <<< FAILURE!
java.lang.AssertionError: Did not receive message of type class org.opendaylight.controller.cluster.raft.messages.AppendEntries
 	at org.opendaylight.controller.cluster.raft.utils.MessageCollectorActor.expectFirstMatching(MessageCollectorActor.java:147)
 	at org.opendaylight.controller.cluster.raft.utils.MessageCollectorActor.expectFirstMatching(MessageCollectorActor.java:125)
 	at org.opendaylight.controller.cluster.raft.PreLeaderScenarioTest.testUnComittedEntryOnLeaderChange(PreLeaderScenarioTest.java:56)

The MessageCollectorActor does receive the message but occasionally out of order
w.r.t the CLEAR_MESSAGES although the latter is sent prior. The problem is that
AppendEntries is a ControlMessage so can get delivered before CLEAR_MESSAGES thus
CLEAR_MESSAGES needs to also be a ControlMessage.

Another problem with many of the tests is that MessageCollectorActors are created
as TestActors and thus get the CallingThreadDispatcher by default which really isn't
desirable. Some of the tests specifically set the default fork-join dispatcher but
we should just create MessageCollectorActors as normal actors.

Change-Id: I455a500d39aae4c46fb33d59be24c6327a9d321d
Signed-off-by: Tom Pantelis <tompantelis@gmail.com>