Add and remove instance/node locks

Whenever we add an instance or node to the cluster (i.e. to the config
and whenever we remove them we should add/remove locks as well). In the
future we may want to optimize this so that the configwriter does it, or
it's handled at the context level, but till we're adding/removing
instances and nodes with the BGL held it doesn't matter too much.

Reviewed-by: iustinp