FileStore: Changed LFNIndex _lookup signature

There is no need every derived class to implement this fast lookup path.
So, made it a non pure virtual.

Signed-off-by: Somnath Roy <somnath.roy@sandisk.com>
Test: FileStore test classes are changed to work with the new Index
and interface changes.

Signed-off-by: Somnath Roy <somnath.roy@sandisk.com>