Add an implementation of RecordStore (primaries) for WiredTiger.

--HG--
rename : src/mongo/db/storage/wiredtiger/wiredtiger_btree_impl.cpp => src/mongo/db/storage/wiredtiger/wiredtiger_index.cpp
rename : src/mongo/db/storage/wiredtiger/wiredtiger_btree_impl.h => src/mongo/db/storage/wiredtiger/wiredtiger_index.h