Avoid sending framework updates to agents and subscribers when frameworkInfo/pid didn't change.

Currently, FrameworkInfo is broadcast to agents and V1API events subscribers on every framework resubscription/update, regardless of whether it has actually changed or not.

When schedulers frequently call UPDATE_FRAMEWORK to only update subscription settings (the list of suppressed roles, and, after implementing https://issues.apache.org/jira/browse/MESOS-10161, offer constraints), the FrameworkInfo broadcast on every UPDATE_FRAMEWORK call becomes undesirable.

Sending 'pid' update to agents is affected by the same issue.