1.6 fixes

Several fixes for 1.6 branch:

• [fix] #1234 (closed) Better CPU usage brought a new bug on PoW
• [fix] #1232 (closed) powCluster.js process still alive after Duniter stop
• [fix] #1233 (closed) Unit tests were reveling "EventEmitter" leaks
• [fix] Unit tests were reveling "unhandledRejection" in proof.ts
• [fix] WS2P Private priority was sometime not respected
• [enh] Remove unecessary "SIGTERM" logs
• [enh] Untrack generated .js files produced by transpilation

I will make another MR specifically for Node8 plug command fix.

cc @nodes/typescript