This library no longer supports Python 2, thus usage of six can be
removed. This also removes workaround about pickle library used in
Python 2 only.

Change-Id: I19d298cf0f402d65f0b142dea0bf35cf992332a9

Since this module contains more than the logbook
class and really is a our generic models that are used
to hold the runtime structure it is more appropriate to
place it under a models module and deprecate the usage
of the old module by placing a warning there (so that
when it is imported that warning is triggered).

Change-Id: I79def5ee08f560d38f2c9dcefd0b33becc2a4d36

When no path is provided to a path based backend via configuration
use a class constant to provide the default, and override this in
backends that support providing defaults.

Change-Id: I0a6c88398403a162b113e34abe7e56821d1f02bc

Change-Id: I6891d53389f302f104d45d22e489cf66feb85fd8

The local conf variable populated by the parent class
already ensures the none/falsey case and sets the conf
to an empty dictionary (as well as checks if its the
expected type and more...); so we should just use that
variable instead of duplicating that work.

Change-Id: I824263d5253d428f6a803f470e29d99de3dbf42c

You can now specify a "lazy" argument when asking a
persistence backend for a logbook for flowdetail.
This will return a logbook with no flowdetails in it,
or a flowdetail with no atomdetails in it, respectively.

Change-Id: Ie81a19a2107b44e2c35c2a92507bec03eee55c50

Factors lots of duplicate code out of persistence backends
Adds get_flows_for_book to all backends

Change-Id: I0434bd4931cd9274876f9e9c92909531f244bcac