Simplify ip allocation/recycling to relieve db pressure

I found that multiple calls to delete_port can pile up on the
_recycle_ip operation.  This patch simplifies this operation.  It
reduces the _recycle_ip operation to a single row delete in the ip
allocations table and doesn't touch the availability table.

To acheive the recycling of ips in a pool, this code runs a more
complex operation of rebuilding the availability table when it is
exhausted.  Only one API process will perform this more expensive
operation and others waiting for allocation will immediately benefit.
The amortized cost of this operation is much less than the cumulative
cost of running the more expensive _recycle_ip operation for every
port delete.

IP allocation behaves a bit differently with this patch.  Instead of
giving out the first IP available in a pool, the entire pool will be
allocated before wrapping around and recycling ip addresses that have
been released.  This is a desirable feature as it puts ip addresses in
a sort of quarantine after they are released.  It is easier to
distinguish newly allocated ips from old ones.

Change-Id: Ia55b66128de9986e075b0f87acc401d211cd91d3
Closes-Bug: #1252506
Closes-Bug: #1257815
(cherry picked from commit 89f25623a4345b5044a58cff36e7fd103c6b88e8)
Refactor to remove _recycle_ip

Since _recycle_ip is now just a pass-through to _delete_ip_allocation
it can be removed.

Change-Id: Ifba3da902de599f748038a33ef3bd98ff77c22b9
Closes-Bug: #1269501
(cherry picked from commit 212c755c34c195148c668b553ccd9f371df63420)
Merge "Simplify ip allocation/recycling to relieve db pressure" into stable/havana
Merge "Refactor to remove _recycle_ip" into stable/havana