Fix Taurus may emit events with outdated data

Current implementation may lead to emitting events with the outdated values. The polling mechanism reads the Tango attributes asynchronously (first drop the read requests and later on collect the replies).

The more up to date event value may get overriden by an older value corresponding to the polling request dropped prior to the event reception.

Implement a filter at TangoAttribute level to drop older events.

This filter can be activated/deactivated via a variable in TaurusCustomSettings

Fix #216 (closed)