tor-spec: Prop#289: RELAY cell padding should be randomised

   drop the cell.  Since more cell types may be added in the future, ORs

   should generally not warn when encountering unrecognized commands.

   The cell is padded up to the cell length with padding bytes. Senders

   SHOULD set padding bytes to NUL and receivers MUST ignore their

   value.

   The cell is padded up to the cell length with padding bytes.

   Senders set padding bytes depending on the cell's command:

      VERSIONS:  Payload MUST NOT contain padding bytes.

      AUTHORIZE: Payload is unspecified and reserved for future use.

      Other variable-length cells:

                 Payload MAY contain padding bytes at the end of the cell.

                 Padding bytes SHOULD be set to NUL.

      RELAY:     Payload MUST be padded to PAYLOAD_LEN with padding bytes.

                 Padding bytes SHOULD be set to random values.

      Other fixed-length cells:

                 Payload MUST be padded to PAYLOAD_LEN with padding bytes.

                 Padding bytes SHOULD be set to NUL.

   We recommend random padding in RELAY cells, so that cell content is

   unpredictable. See proposal 289 for details. For non-RELAY cells, TLS

   authenticates cell content, so randomised padding bytes are redundant.

   Receivers MUST ignore padding bytes.

   PADDING cells are currently used to implement connection keepalive.

   If there is no other traffic, ORs and OPs send one another a PADDING