add lastEvent timestamp to event

[Ingvord]

We could have a request like this:
"https://mstatus.esrf.fr:443/tango/rest/rc4/hosts/tangorest01.esrf.fr/10000/devices/sys/mcs/facade/attributes/state/change?timeout=30000&lastEvent=1507277908»
… which would return the first event received after the provided timestamp (if any), or wait until the timeout expires.

[Ingvord]

Sounds reasonable to me. This can be done in rc4 as it does not break compatibility

[Ingvord]

As actual request is more complicated then just providing a single value but a buffer I reopen this one.

• What we need is to add a new end point: attr/change/buffer?[limit=100]
• Define Tango server attribute - maxEventBuffer