Generalize BMv2 so that it can apply the same table multiple times

[jafingerhut]

i.e. multiple times during a single execution of the ingress control on a single packet, or multiple times during the same execution of the egress control on a single packet.

• Applying the same table twice behavioral-model#1237

I believe that this would require:

• generalizing the data model of a BMv2 JSON file in a way that supports a table being applied multiple times, e.g. perhaps there should be some kind of state machine, where each state can refer to one or more tables to be applied, and the state machines have links to next-possible-states. In the current BMv2 JSON file format, tables have next-possible-table fields, which is the root cause of the current restriction.
• generalizing BMv2 to be able to read and process packets according to this new BMv2 JSON data model.
• modify the p4c BMv2 back end to generate BMv2 JSON files with this new data model.