gentable infrastructure

Reviewer: @poolakiran

See https://github.com/floodlight/loxigen/pull/165 for the OpenFlow extension. This code provides infrastructure that make implementing gentables very easy - just write 4 functions and register the table during init. Most of the code here is stats boilerplate or checksum magic that we'd have to reimplement for each table if it weren't in common infrastructure.

Please carefully review the checksum iteration / long running task code.