[ESI] [Cosim] DPI cosim Capnp server polling

https://github.com/llvm/circt/blob/efd420ec9cf402b341c027fe9c435c8c29ab5912/lib/Dialect/ESI/cosim/cosim_dpi_server/Server.cpp#L207

Currently, the capnp DPI server polls for events in order to "listen" for the stop signal. I spent some time trying to get this working properly, but this hack is the best I could do. Ideally, the simulation thread would send a signal to the cosim capnp server through some libkj mechanism. Less ideally but better than what we've got, one would run the libkj event loop with a timeout.

More recently, libkj was documented: https://github.com/capnproto/capnproto/blob/master/kjdoc/tour.md
and a question was asked on how to run the event loop with a timeout: https://groups.google.com/g/capnproto/c/BU8olURJ6fE/m/Do117V12AQAJ?pli=1

Start at lib/Dialect/ESI/cosim/cosim_dpi_server/Server.cpp around line 207, where the capnp DPI server polls for the stop signal. Read the linked libkj event-loop documentation and timeout discussion before choosing an approach. Done means the server can observe the simulation stop signal without the current polling hack.

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