Get rid of useless (Hwvtep)SouthboundProvider thread

Get rid of useless (Hwvtep)SouthboundProvider thread

The thread instantiated here is useless and the comment is misleading:
ListenerRegistration.close() is specified to be safe in any context
and idempotent. Just get rid of the thread and close the registration
immediately.

This flushes out an immediate problem, where the test would deadlock
due to base test using directExecutor() for DTCLs -- i.e. simulating
execution which does not happen in production. To deal with that we
switch the test to use asynchronous DTCLs and add a simple synchronization
loop to wait for DTCL to trigger.

JIRA: OVSDB-454
Change-Id: I2e86acb9eb30529fcd5d5675005d6c81bd804ba5
Signed-off-by: Robert Varga <robert.varga@pantheon.tech>