MDS-321: check, a part name doesn't start with '/'

If a part name starts with one or several '/' proxy will remove them,
e.g. '///partname123' will become 'partname123'. But if the part name
only consists of '/' (e.g. '///////') the result will be 400 Bad Request
MDS-322: mds-proxy may recover a file

Mds-proxy during processing get handler may send WRITE command and
recover a file with following restrictions:
1. User's request causes a reading of whole file. (Even if request has
Range header proxy may read whole file, e.g. see If-Range header for
details.)
2. The size of the file less than or equal to read-chunk-size. That
means the file will be read by one READ command.
3. Quorum-lookup returns either one or more groups with statuses which
equal to -ENOENT. And WRITE command will be sent only in that groups.
Version up to 0.74