FileStore: Improvement in LFNIndex lookup logic
The string is preallocated in the LFNIndex::lookup and sent as reference
to the subsequent function call. The stat is been replaced by access()
since it should be cheaper. HashIndex::_lookup is been overridden and the
logic is optimized in the overloaded function. Same with
LFNIndex::lfn_get_name(). The string computation logic within
lfn_generate_object_name is been optimized as well.

Signed-off-by: Somnath Roy <somnath.roy@sandisk.com>
FileStore: Indexmanager lookup optimized
Earlier in get_index() the map.count() followed be map[] operator was used.
This can be expensive since two logarithmic complexity operation.
Now, it is replaced by map.find() to get rid of one logarithmic search.

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