XD-1490 support absolute path in CacheServer
XD-1580: Undeploy modules when ZK disconnects

In method registerWithZooKeeper, the module deployments cache is
started before saving the container attributes. This way if there
is an exception thrown while saving the attributes, the deployments
cache is not affected.

At some point we may want to consider trapping exceptions from
this method, or perhaps even shutting down the ZK connection if
an exception occurs. I was seeing a NodeExists exception for
the ephemeral node created by this method, but that exception
seems to have gone away with the other changes in this commit.

In method onDisconnect the modules are now undeployed when the
connection to ZK is lost. This is desirable because the admin
leader will try to redeploy those modules on another container.

In StreamModuleWatcher and JobModuleWatcher, instead of blindly
setting the watch on every non-delete event, the watch is only
set if the connection to ZK is established. This change seems
to prevent the exception described above in registerWithZooKeeper
although I don't understand how they are related.