Implement signals for adding plugin hook support in nodeos

[andresberrios]

This adds the required signals for the proposed nodeos statetrack_plugin to hook into. Another pull request has been opened at https://github.com/EOSIO/eos/pull/6321 to review that implementation.

Details about the approach and open questions can be found in the readme file of the plugin: https://github.com/mmcs85/eos/tree/master/plugins/statetrack_plugin

Details about how the operations should be handled by a receiver can be found in the statemirror repository: https://github.com/andresberrios/statemirror