Use string for delay and packet loss parameter, code refactoring,
improved error messages, fix typos, add empty status representation,
fix namesapce ID representation.
To avoid to run `tc` every tic to get the qdisc status, we cache it.
The cache is updated every time a value is changed via the control interface.
NOTE: This will keep track only of the changes by done the plugins interface,
if the qdisc parameters are changed by another applcation or by the user
via terminal, such changes will not be caught.
This patch sends the delay and packet loss values along with
the container metadata. In this way the user has a direct feedback
about the traffic control plugin status.
As it is an initial implementation, it only controls latency of the
outgoing (egress) traffic. There is also a TODO to turn this plugin
into something more serious. Also, at some point we may consider
moving this plugin outside of "example" directory.
