Dynamic labels on the master node are broken:

• the first check is done when toComputer() returns null; the result is cached
  and no later checks are performed
• lots of code is duplicated with hudson.model.Slave.

The patch I'm attaching fixes this by:

• consolidating the two connected variables dynamicLabels and
  dynamicInstanceHash into a single instance of a new class, fixing race
  conditions between the two and making code dealing with them cleaner
• pulling the common code between Hudson and Slave for dealing with dynamic
  labels up into Node

I hope the style etc is ok, please let me know if you need more changes done to
be able to land the patch. I could see a bunch of improvements in the labelling
area but I didn't want to make a bigger patch than needed to fix what we want to
do in the squid project (where we want to tie jobs to the arch-os-version of
nodes and we are entering it by hand at the moment).

This patch would allow the OSLabeller to be enabled (I used it as a testbed),
but it doesn't meet our project needs, and enabling it is a separate conceptual
step - thus its been left disabled.