Improve file handling in serializers

[Natalia Bidart]

As per facundobatista's review on lp:~nataliabidart/wsgi-oops/serializer-thread-safe, we should improve file handling using a manager like this one (untested code):

class FHManager(object):
    def __init__(self, instance_fh):
        self._instance_fh = instance_fh
        self._fh = None

    def __enter__(self, filename):
        if self._instance_fh:
            return self._instance_fh

        if os.path.isdir(filename):
            filepath = os.path.join(filename, section)
            fh = open(filepath, "w")
        else:
            filepath = filename
            fh = open(filepath, "a")

        self._fh = fh
        return fh

    def __exit__(self, type, value, tb):
        if self._fh:
            self._fh.close()

And then, within the file serializer, make a cal like this one:

with self.fh_manager(filename) as fh:
    ...