"sorted" usage should be eliminated during get-next calls

[tombo315]

Ideally (as-implemented):

• O(1) for gets
• O(n) for get-next

Currently:

• O(1) for get
• O(n) for gets in dynamic subtrees Implement RFC4363 dot1qTpFdbTable Q-BRIDGE-MIB #3
• O(n log n) for get-next (because of sorted usage)

We can have the table maintain a sorted copy of the keys for fast get-next calls.

class MIBTable(dict):
   # ...
  def __setitem__(self, key, value):
      super().__setitem__(key, value)
      self._sorted_keys = sorted(self.keys())

  @property
  def sorted_keys():
      # make a copy -- saves OID searches "in-flight" if the table is updated
      return list(self._sorted_keys)