Increasing numprocesses often causes some processes being killed

[sergiimk]

Scenario:

• configure a watcher with numprocesses = 1 and warmup_delay = 1
• start circus
• change numprocesses = 5
• send circusctl reloadconfig command

Expected: circus spawns 4 more processes

Actual: 7 more processes spawned, 3 processes killed (sometimes varies)

Already traced down the root cause of this problem, PR coming soon.