dashboard: where a stream's lag comes from

A stream's lag in the tooltip of `behind: …` was one figure, from when
a node logged a message to when the page had it: a second and a half
for a node a LAN away said nothing of where it went.

The node now stamps each `messages` event with when it sent it
(`{"sent": …, "messages": […]}`), and the tooltip splits each stream's
lag: how long its batches waited in the node, from logging the first
message to sending the batch (the node's own clock: exact), and how
long they took on the way, from sending to arriving. This node's own
stream is always listed, beside the three slowest.

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