Add an option to run rootwrap as a daemon

Benchmark for daemon mode is to be added in next commit.

Current results on my machine for 'ip a':
        method          :    min       avg       max       dev
                   ip a :   3.701ms   4.966ms  12.720ms   1.206ms
              sudo ip a :  11.531ms  13.538ms  19.685ms   1.525ms
sudo rootwrap conf ip a :  92.253ms 119.431ms 211.236ms  17.948ms
     daemon.run('ip a') :   6.718ms   9.914ms 189.873ms  18.201ms

Implements: blueprint rootwrap-daemon-mode
Change-Id: Iace26738f910a18a5d1d3479fad949027e5a3816
Merge "Add an option to run rootwrap as a daemon"