Fix last_modified_date_to_timestamp on non-UTC systems

Before, we were calling datetime.datetime.strftime('%s.%f') to convert
a datetime to epoch seconds + microseconds. However, the '%s' format
isn't actually part of Python's library. Rather, Python passes that on
to the system C library, which is typically glibc. Now, glibc takes
the '%s' format and helpfully* applies the current timezone as an
offset. This gives bogus results on machines where UTC is not the
system timezone. (Yes, some people really do that.)

For example:

    >>> import os
    >>> from swift.common import utils
    >>> os.environ['TZ'] = 'PST8PDT,M3.2.0,M11.1.0'
    >>> float(utils.last_modified_date_to_timestamp('1970-01-01T00:00:00.000000'))
    28800.0
    >>>

That timestamp should obviously be 0.

This patch replaces the strftime() call with datetime arithmetic,
which is entirely in Python so the system timezone doesn't mess it up.

* unhelpfully

Change-Id: I56855acd79a5d8f2c98a771fa9fd2729e4f490b1
Merge "Fix last_modified_date_to_timestamp on non-UTC systems"