Trace API Plugin and configuration scaffolding

[b1bart]

In this PR:

• basic plugin scaffolding for (2) plugins
  • a full system plugin _trace_api_plugin (temp name until the namespace is renamed)
  • a plugin that only serves the RPC api trace_api_rpc_plugin
  • only the full system will be linked in and available to nodeos
  • the rpc plugin is laying the ground work for an independent process and to force separation of internal logic
• new header and test with what little testable logic fell out of configuration code
• new concept for adding context to exceptions so that logging appropriately correlates messages to the code that produced them. There is an example of how this is wired in through the abi_data_handler so that the throwing code can provide the context to the injected log function.

[heifner]

Approved pending slack discussion.