Fix high CPU usage when running the SDK server

It was coming from `recv_select()` `nd accept_select()`
The timeout for select() is only when waiting for the file descriptor to
be ready for reading/writing so when the FD is ready AND there is
nothing to read, it will not return and instead keep executing the while loop with no timeout.
Fix this by adding a 100ms timeout when there is nothing to read.
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Gitea Version: 1.28.0+dev-477-g8b6ad49a5f