Add importable SwiftService incorporating shell.py logic

This patch adds a SwiftService class that incorporates the high
level logic from swiftclient/shell.py. It also ports shell.py to
use the new class, and updates the code in swiftclient/multithreading.py
to allow the SwiftService to be used for multiple operations whilst
using only one thread pool.

Currently, code that imports swiftclient has to have its own logic for
things like creating large objects, parallel uploads, and parallel
downloads. This patch adds a SwiftService class that makes that
functionality available in Python code as well as through the shell.

Change-Id: I08c5796b4c01001d79fd571651c3017c16462ffd
Implements: blueprint bin-swift-logic-as-importable-library