Changed context merging logic from SaveWithBlock methods

Merge all changes from Root saving Context in default context can cause
unwanted behaviours if the changes are initilally generated in default
context itself (Default Context -> Root Context -> Default context). In
many cases deleted MO “reappears” in the default context after saving
with MR_saveToPersistantStore methods.
For this reason I’ve removed the observer from the root context and
added an observer only for localcontext created in saveWithBlock methods