Repeated on-demand vhds updates unsubscribe from previously resolved vhosts

Description:
If a vhost named "foo.com" has been previously resolved, an attempt to resolve "bar.com" will result in "foo.com" vhost being unsubscribed from, and all updates for it lost.

This is due to how `GrpcMuxWatch::update` is implemented -- it expects a list of resource names that should be watched. The resources which names are not in the list will be unsubscribed from.

edit: not an issue for NewGrpcMuxImpl::addWatch