avocado.multiplexer: Support for debug run [v1]

Hi guys,

there are bunch of improvements and support for debugging of the multiplex files. See the commit messages for details.

I did some comparison regarding the environment caching and when using `examples/*.yaml` the results were 10x quicker (10^-4 vs. 10^-5). First query took about the same time, every other was more than 10x quicker. The memory consumption were minimal.

About the debug version, results also seems all right:
```
original version: ~160KB
Function <function create_from_yaml at 0x2351aa0>: (0.00369906425476, 0.00369906425476)
Function <function create_from_yaml at 0x1d63aa0>: (0.00297999382019, 0.00297999382019)
Function <function create_from_yaml at 0xcd6aa0>: (0.00528192520142, 0.00528192520142)


version with debug, debug disabled ~160KB
Function <function create_from_yaml at 0x2316cf8>: (0.00302505493164, 0.00302505493164)
Function <function create_from_yaml at 0x12abcf8>: (0.00411605834961, 0.00411605834961)
Function <function create_from_yaml at 0x2807cf8>: (0.00311088562012, 0.00311088562012)


version with debug, debug enabled ~700KB
Function <function create_from_yaml at 0x12e5cf8>: (0.00979804992676, 0.00979804992676)
Function <function create_from_yaml at 0x256fcf8>: (0.0095489025116, 0.0095489025116)
Function <function create_from_yaml at 0x1063cf8>: (0.0103480815887, 0.0103480815887)
```

you can see that normal run is +- unaffected which was my goal and I hope it would add to the nasty code which was required in order to get the debug working without affecting the normal runs.

Regards,
Lukáš