Expand and test constant names.

* Purge exec flag from constants.py
* Define the maximum number of contacts per replacement cache.
* Define HEX_NODE_LEN_ID as the length of a node ID in hex.
Refactor node ID processing.

* Add and test numToId for easy conversions.
* Refactor _randomIDInBucketRange to use numToId.
* Refactor distance to use node ID length, and raise
  an exception when either of the keys is of improper
  length.
Test and refactor getRefreshList.

* Be deterministic; do not use randomized IDs.
* Drop _randomIDInBucketRange as it is no longer used.
* Use a list comprehension for simplicity.
* Avoid invoking time.time() many times. :P
* Optimization: do not enter loop if force flag is set.
* Drop unused import.
Drop addContact from TreeRoutingTable.

* Its proper implementation is contested.
* It references undefined names ("addErrback").
* The class itself is nowhere used in the code.
* It is very difficult to test, as it entails the network.