RequestCache cleanup.

- Identifiers are now unicode strings, these are used as keys in the
  RequestCache instance and as Callback id's.

- Cache now takes a unicode string as a parameter, this is the
  identifier.  Older code set the identifier (outside of the Cache
  constructor) once the cache was added to the RequestCache.

- NumberCache is used when, instead of a predefined unicode string,
  the identifier must contain a random number.  This is used by, for
  instance, the introduction-request message.  The random number is
  available from the 'Cache.number' property, while the
  'Cache.identifier' property still contains the unicode string.

  Note that this means that any older code that used to encode
  Cache.identifier into a request or response message must now use
  Cache.number instead.

- The static method Cache.create_identifier is used to generate a
  Cache specific unicode string.  Parameters can be supplied to add
  unique features, such as the number in NumberCache.

  Note that the generic method 'handle_missing_messages' needs to get
  the identifier from a message instance.  This is done using
  Cache.create_identifier_from_message, which in turn should call
  Cache.create_identifier with the appropriate parameters obtained
  from the message instance.  Code that uses 'handle_missing_messages'
  must ensure their Cache object supplies
  'create_identifier_from_message'

- RequestCache.add(CACHE) replaces RequestCache.set(ID, CACHE).
  RequestCache.add will return None when CACHE.identifier is already
  in use.

- RequestCache.claim no longer exists.  A valid (read unused)
  identifier can be found using RequestCache.has or repeated calls to
  RequestCache.add, while checking its return value.

- RequestCache.{replace, has, get} have had their parameters modified
  to reflect the introduction of the unicode string identifier.