Improve vxlan type driver initialization performance

Vxlan type driver may take long time to initialize
vxlan allocation table. Optimize db performance by issuing
raw sql inserts coalesced into bulk statements.
Also optimize deleting logic.

Proposed patch gives ~2x performance gain in comparison with
original code on Mysql and Postgesql backends

Change-Id: I801d967e8e3c0260593f289097d17270ef0b391e
Partial-Bug: #1324875
Merge "Improve vxlan type driver initialization performance"