The esp8266 uses modlwip.c for its usocket implementation, which allows to
easily support callbacks on socket events (like when a socket becomes ready
for reading). This is not as easy to do for the esp32 which uses the
ESP-IDF-provided lwIP POSIX socket API. Socket events are needed to get
WebREPL working, and this patch provides a way for such events to work by
explicitly polling registered sockets for readability, and then calling the
associated callback if the socket is readable.
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