add total latency to perf charts

[Antoine Martin]

As part of #619 and #2130, it would help to be able to see how long screen updates take to land on the client's screen.
In the same way that we have the "overall latency" on the session info graphs.

[Antoine Martin]

r23539 records two new attributes in xpra info:

• client.damage.frame-total-latency: how long it takes from the moment we get a damage event until we get the draw acknowledgement packet from the client, that's everything we do - which can be summarized as: batch delay + compression + sending + network latency + receiving + updating the window buffer + sending ack + network latency (again) + receiving ack
• client.damage.client-latency: same, minus the connection latency (note: this latency includes the cost of the protocol layer - which is small in most but not all cases..)

As of r23541, these new attributes are now exposed in the perf test data as:

• "Frame Total Latency"
• "Client Frame Latency"

@smo: does that work?
Maybe re-run just NODELAY=0 + CORK=0 vs default settings.

[Smo]

Added these to charts in r23546

[Smo]

nodelay0 cork0 vs defaults

[Smo]

Here are the results attached to the ticket.

Something looks off to me what do you think?