Original title: The emptycapped command fails to restore indexes for non-capped collections

In 2.7.0+, the emptycapped command fails to restore indexes for non-capped collections because truncate() is not implemented in SimpleRecordStoreV1. The command is also not replicated, which leads to inconsistent indexes across the primary and secondaries.

test_commands.cpp:

203    std::vector<BSONObj> indexes = stopIndexBuilds(txn, db, cmdObj);
204
205    Status status = collection->truncate(txn);
206    if ( !status.isOK() )
207        return appendCommandStatus( result, status );
208
209    IndexBuilder::restoreIndexes(indexes);
210
211    if (!fromRepl)
212        repl::logOp(txn, "c",(dbname + ".$cmd").c_str(), cmdObj);