[#176] blobstor: Use 1 byte names of sub-directories in FS tree

Using 2 bytes length for directories in fs tree lead to large growth
in the number of directories.

Signed-off-by: Leonard Lyubich <leonard@nspcc.ru>

pkg/local_object_storage/blobstor/fstree.go

@@ -20,7 +20,7 @@ type fsTree struct {
 	rootDir string
 }
 
-const dirNameLen = 2 // in bytes
+const dirNameLen = 1 // in bytes
 
 var maxDepth = (sha256.Size - 1) / dirNameLen