How posix_spawn & close_range “fixed” cold starts

The Pageserver in Neon's system experienced a severe regression in cold start latency after making changes to improve stability. The root cause was found to be the inefficient closing of file descriptors as part of the defense-in-depth strategy against compromise of walredo processes. To fix this, the code switched from using the `close_fds` crate to manually issuing the `close_range` system call, which allowed for faster process creation and reduced latency. This highlights the importance of dynamic tracing at runtime and the need to carefully consider the trade-offs in process creation primitives.

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