Move postcommit ops out of transaction for bulk

Move postcommit ops out of transaction for bulk

Currently, the bulk create operations in ML2 are executed in
a transaction. This means all precommit and postcommit operations
for such operations are in a transaction. Postcommit operations
are expected to be executed outside of transactions as they may
communicate with a backend and introduce substantial delays. This
fix removes the postcommit operations from the transaction for
bulk create network/subnet/port operations.

Change-Id: I9a9683058088e50d9443040223232bf5e1396ccf
Closes-Bug: #1193861