[ingvarch]

What changes

A screen that opened used to start the event stream from the start of the
cluster's event buffer. The buffer sent again every change it held, and the
screen read its list once more for each of them: a list was read two or three
times when it opened. Now each read notes the index it read at, and the stream
starts after it. A change the screen already shows does not read it again.

• Every read notes the index of each topic it read through its request
  context (nomad.WithReads, NoteRead), so no signature in internal/nomad
  changes. Events takes the index to start from, and each change carries its
  index.
• A screen keeps one read index per topic. It opens its stream on its first
  answer, from the lowest of them plus one, and drops a change at or below
  the index of its topic.
• An answer older than the page, which comes after a newer one, makes the
  screen read again: a change dropped against the newer index is not lost.
• The client list reads allocations only when a client has a limit. Without
  a limit its allocation index is EveryIndex, and a change dropped in that
  time is kept. When a client gets a limit, the screen reads again if the
  kept change is newer than the new read.
• The jobs list keeps the index of the list. The index of the counts of the
  services is the newest service on the page, which made the list read twice
  when it opened.
• A read that failed notes no index. This includes a part of the overview
  that failed after its list answered, and a queue with a pool that did not
  answer.
• Once the stream opens after the answer, the poll timer restarts at the slow
  pace.
• docs/configuration.md says which screens can still read once more when
  they open.

Left for later: a screen of one object (a job, a client) still reads again
when something else it follows changed after that object did. #189 tracks it.

Closes #165

Checklist

• The test for this change was written first and failed
• make check is green
• The title is a conventional commit: feat(ui): ..., fix(nomad): ...