Add comments and test cases:
 * dots are used as separator in buffer paths
 * for keys, dots are also used
 * in resume sequence, file buffer plugin cannot determine whether:
   * "any.chars" is key
   * "__prefix__.any" is the prefix of other buffer plugin instance
   * for the path such like "__prefix__.any.chars.b000000.__suffix__"
 * Hopefully, we should escape dots (".") in keys...
   * But it breaks compatibilities