This reduces considerably the number of nodes created and thus speeds
up the calls to node_get.

Calls to `isinstance` can be particularily costly. Using type() is
much faster. The only known case where the `isinstance` was useful
is for dictionnaries where we would ask for a 'Mapping' instead.

Disallowing 'Mapping' for expected_type considerably speeds up the
calls to this functions.

Also add into NEWS

setup.py: Be more restrictive with BST_CYTHON_TRACE values

See merge request BuildStream/buildstream!1368

"0" evaluates to 'True' in python, which incorrectly switched on
the BST_CYTHON_TRACE. Forcing an int for this environment variable
allows us to ensure we are correct.

Setup.py: Disable linetrace by default and only enable when using coverage

See merge request BuildStream/buildstream!1367

It turns out that enabling 'linetrace', does have a runtime cost even
if not enabled in distutils.

Therefore disabling it by default. In order to run coverage, we
therefore need to retranspile the .pyx files with ENABLE_CYTHON_TRACE
set.

Update docs regarding artifact and source caches

Closes #1024

See merge request BuildStream/buildstream!1362

Now that we have both artifact and source caches the documentation
is updated to reflect that. Some sections headings/links etc. have been
expanded and changed.

Part of #1025

Part of #1024

Part of #1024