Fix Task.yield_many/2 waits too long or forever when :limit exceeds task count

[AlexGx]

Task.yield_many/2 waits until timeout/forever when :limit exceeds number of unique tasks.

Examples:

task = Task.completed(:ok)

Task.yield_many(
  [task, task],
  limit: 2,
  timeout: :infinity
)
# waits forever, unique task count is lower than limit

task = Task.async(fn -> :done end)

Task.yield_many(
  [task],
  limit: 2,
  timeout: 15_000
)
# waits 15 secs, instead of returning immediately

The fix is to clamp limit to the number of unique task refs so the function returns as soon as all tasks have been completed.

[josevalim]

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