Cached misses in getMulti should be omitted

Cached misses in getMulti should be omitted

SqlBagOStuff::getMulti currently returns an array with false values for missed
cache keys.
Default BagOStuff::getMulti (and other specific implementations like memcached),
however, just omit missed values.

e.g.:

    $cache->getMulti( array( 'goodkey' 'badkey' ) );

should return

    array( 'goodkey' => 'value' );

instead of

    array( 'goodkey' => 'value', 'badkey' => false );

Bug: 65637
Change-Id: Ib2de06a0f76421094707f7ad4218346087f4dc55