Avoid long transaction in plugin.delete_ports()

Avoid long transaction in plugin.delete_ports()

db_plugin.delete_ports() called plugin.delete_port() under
a transaction. It leads to long transaction if plugin.delete_port
talks with external systems. This commit changes each delete_port
outside of a transaction to avoid longer transaction.

plugin.delete_ports is now called by release_dhcp_ports and
dhcp-agent ports can be deleted separately, so this changes
does not break the existing behavior.

delete_ports is renamed to delete_ports_by_device_id
to clarify the usage of this method.

NEC plugin already has this change and it is no longer needed.

_do_side_effect helper method in test_db_plugin is renamed
to more self-descriptive name.

Change-Id: Ied5883a57c7774c3b0778453d84c717b337f88c0
Closes-Bug: #1282925
Related-Bug: #1283522