WIP: Fix #53: Add validation for $set, $unset Update operations not to allow changing _id

[tatu-at-datastax]

Changed the way Update operations are handled: instead only having UpdateOperator and DocumentUpdater implementing logic there is now base type UpdateOperation and construction happens in 2 separate phases:

1. Simpler deserialization from JSON into UpdateClause; binds UpdateOperator marker but not JSON Object that configures operation
2. UpdateClause.buildOperations() will construct UpdateOperations and handle cross-operation validation -- this will allow passing more configuration if necessary, in future (for Upsert handling etc)

With the logic separated more validation is done:

1. We will check if Update operator is valid (starts with "$"); and if so if we actually support it (added $inc as an example of known but not yet supported)
2. "$set" and "$unset" must not be used on _id (now checked)
3. "$set" and "$unset" must not be used for same path (cannot both set and unset 'x' etc)

This change is the base to start adding handling for nested properties (sub-documents).
Could also consider implementing "$inc" operator since it seems relatively straight-forward.

[maheshrajamani]

Looks good to me.