The VLAN and tunnelled network segment range initialization is performed
in each Neutron API worker during the initialization transient period.
This patch adds the following optimizations:
* The ``network_segment_ranges`` expired register deletion is based on
the Neutron API start time, that is unique for all workers. The new
registers created by other API workers won't be deleted.
* A new method ``NetworkSegmentRange.new_default`` is used to create
the new default registers that is process-safe, using the database
transactional engine.
Closes-Bug: #2086602
Change-Id: Id1551dcd786202384393556e15e91de72aa7272e
d3bc0b920b