Use absolute path for .fileindex in osrm-datstore

Currently the following procedure leads to error:
 1) build/osrm-datastore test/cache/sample1.orsm
 2) cd some_other_dir
 3) orsm-routed --sharedmemory=yes

osrm-routed does not fine the sample.fileindex file because it tries to use given relative path:
`test/cache/sample1.orsm`
If the file name will be stored in memory as absolute path, user can run osrm-datastore and osrm-routed from arbitrary folder without giving the full path to .osrm.