elanmanager: use transaction manager

elanmanager: use transaction manager

This replaces all direct invocations of
DataBroker::newWriteOnlyTransaction() with calls to a
ManagedNewTransactionRunner, ensuring all such transactions are
closed. It also reworks a number of methods to use read/write
transactions, ensuring that changes are made based on still-relevant
data.

This doesn’t address transactions in ElanInterfaceManager.

Change-Id: Id38c7482cf2403a7751f0faa6134be7374e4a072
Signed-off-by: Stephen Kitt <skitt@redhat.com>