[API][ADMIN_API] Fix small race in SQLAlchemy + galera

It is possible for a session to be cleared before galera replication is
done causing a transaction abort.  Now only switch engines for sessions
after 60 seconds of idle time.  This also gives us a 60 second failover
window.

Change-Id: I8db17a13f460d4be580a170d8cfdcbab40eb6b02