Move synchronized body to a first-class function

synchronized is being widely used throughout OpenStack for managing
locks by decorating methods / functions. However, it's functionality is
trapped within that decorator definition and makes it difficult to
manage locks by using it in other cases (like w/o decorating).

This patch moves decorator's body into a first-class function called
lock which is intended to be used as a context manager type (with
statement).

Some examples:

    with lockutils.lock("test") as sem:
        print('Inside the lock')

Things I'm not 100% convinced:

    * The `lock` function yields a Semaphore when external is False and
    an InterProcessLock instance otherwise. Although it is not 'good' to
    yield different values depending on the input parameters, it's
    pretty explicit (by reading the documentation) that external locks
    are handled differently. Other options for this case are:

        1. Always yield a Semaphore instance
        2. Don't yield anything

The patch doesn't break backward compatibility so no change is needed to
existing projects.

Implements blueprint cache-backend-abstraction

Change-Id: I96b2ee84da5a2dcd7f0bd469f8a1e52b74e50c75
Merge "Move synchronized body to a first-class function"