Bug 949325 - C++ wrapper to support DataStore API on the worker (part 1, C++ stubs for DataStore and DataStoreCursor). r=baku,ehsan

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rename : dom/datastore/DataStoreCursor.jsm => dom/datastore/DataStoreCursorImpl.jsm
rename : dom/datastore/DataStore.jsm => dom/datastore/DataStoreImpl.jsm