Remove use of jerkson and use objectnode map.

In order to facilitate re-use of the ZKMap for work units, create one
which maps from a work unit to an ObjectNode. Additionally, move all
JSON parsing from Jerkson / 1.9.x to Jackson 2.1.x.
Add a latch for determining when it is safe to use maps.
Merge pull request #32 from boundary/map_objectnode

Remove use of jerkson and use objectnode map.