Move requests-logging setting to global scope

This should happen at import, since the __init__ may have been run in a different proc, and the cache pickled, since then.

Testing Done:
https://travis-ci.org/pantsbuild/pants/builds/38522554

Reviewed at https://rbcommons.com/s/twitter/r/1022/
The traversal order used is preorder, not inorder; support postorder traversal

1. The term "inorder traversal" does not make sense outside of the context of binary trees.  Instead, use the term "preorder", which is actually what is done.

2. Also allow for postorder traversal (and test that it works).

Postorder stuff is needed for https://rbcommons.com/s/twitter/r/1179/

Testing Done:
./pants goal test tests/python/pants_test/graph:graph

Reviewed at https://rbcommons.com/s/twitter/r/1177/
Handle list-typed options (action='append') correctly.

Ensure that we read them from config in the right way, and apply type
conversion to the list members.

Added test cases for this.

Testing Done:
Ran the options unittests before and after the fix, to verify
that the test discriminates this.

CI passed.

Reviewed at https://rbcommons.com/s/twitter/r/1183/