Logic for handling formula states in ZK is needed

[Gustavo Niemeyer]

Along the lines of what was described in bug #616485 in the context of services, and bug #616487 in the context of machines, we also need state classes to deal with formulas.

The following classes are recommended:

FormulaStateManager: Instances of this class will take care of handling the collection of formulas which are known in the Zookeeper environment. Some example operations would be adding new formulas, finding existing ones, and removing old and unwanted ones.

FormulaState: Instances of this class will deal with details pertinent to a single formula, such as its name, its revision number, and its digest.

Part of this logic is necessary to upload new formulas in the environment, as per bug #616478, and to get formulas deployed, as covered in bug #616490.

The following bugs describe additional features of the FormulaStateManager:

- Bug #619370: Searching for formulas
- Bug #619371: Formula removal
- Bug #619280: Formula replacement