Merge branch 'snakeviz' into 'master'

For more detailed documentation of cProfile and 'pstats', see:

https://docs.python.org/3/library/profile.html.

For a richer visualisation of the callstack you can try `Pyflame

<https://github.com/uber/pyflame>`_. Once you have followed the instructions in

Pyflame's README to install the tool, you can profile `bst` commands as in the

following example:

For a richer and interactive visualisation of the `.cprofile` files, you can

try `snakeviz <http://jiffyclub.github.io/snakeviz/#interpreting-results>`_.

You can install it with `pip install snakeviz`. Here is an example invocation:

    pyflame --output bst.flame --trace bst --help

You may see an `Unexpected ptrace(2) exception:` error. Note that the `bst`

operation will continue running in the background in this case, you will need

to wait for it to complete or kill it. Once this is done, rerun the above

command which appears to fix the issue.

Once you have output from pyflame, you can use the ``flamegraph.pl`` script

from the `Flamegraph project <https://github.com/brendangregg/FlameGraph>`_

to generate an .svg image:

    ./flamegraph.pl bst.flame > bst-flamegraph.svg

The generated SVG file can then be viewed in your preferred web browser.

    snakeviz bst.cprofile

It will then start a webserver and launch a browser to the relevant page.

Profiling specific parts of BuildStream with BST_PROFILE

~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~