Enhance pytest

1. Rename instance.killpg -> kill().

   The intention is to eliminate ambiguity with os.killpg. Make the Instance API more like subprocess module (as regards kill and terminate functions).

2. Log more instance management events:

   Instance(i1, listen=127.0.0.1:3301) starting...
   Instance(i1, listen=127.0.0.1:3301) is ready
   Instance(i1, listen=127.0.0.1:3301) is trying to become a leader
   Instance(i1, listen=127.0.0.1:3301) is a leader now
   Instance(i1, listen=127.0.0.1:3301) terminated
   
   Instance(i1, listen=127.0.0.1:3301) starting...
   Instance(i1, listen=127.0.0.1:3301) killed