Refactor config-persister: clean up exception handling and netconf client.

Refactor config-persister: clean up exception handling and netconf client.

Split ConfigPersisterNotificationHandler into registrator and listener, remove unnecessary
reference to NetconfClient.
Refactor ConfigPusher: each method now returns computational results instead of void. Avoid
unnecessary NetconfClient reference leak - close it after every use. Rename generic 'timeout'
to 'maxWaitForCapabilitiesMillis'. Avoid throwing and catching SAXException and IOException
as there is no recovery from them; keep only ConflictingVersionException.
ConfigPersisterActivator now actually uses the configurable timeout, the property is renamed
from 'pushTimeout' to 'maxWaitForCapabilitiesMillis'.

Change-Id: Idc9995b11dd2f19a90598c88eb7337d39d63fc27
Signed-off-by: Tomas Olvecky <tolvecky@cisco.com>