Limit server memory

[emilk]

What

Limit the amount of RAM used by the WebSocket server.

The WebSocket server will buffer all log data in memory so that late connecting viewers will get all the data.
Previously this would just eat more and more RAM until the server crashed.

There is now a limit, which default to 25% of the RAM. That is, the server will try to keep its own memory use under that limit. Note that this acts different from the viewer's --memory-limit which throws away old data when the total process memory exceeds the limit.

The memory limit is set by the new --server-memory-limit argument to rerun, or by argument to the serve functions. Note that I opted for a different argument for the server limit than for the viewer memory limit. This is partially because it should be a lot less, and partially because the limit acts differently (it is a local budget, rather than a total process limit).

While working on this I noticed we don't have a serve function in C++, so I created an issue for that: #4638

The default limit of 25% can be discussed. It leaves the other 75% for the viewer (though if the viewer is in a browser, it will likely be using much less).

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