Refactor network helpers (Take 3)

[Shourya742]

This PR combines the previously separate reader and writer tasks into a single tokio::select! block. It also adds a SIGTERM handling arm for graceful termination and introduces a writable_len method in the decoder to determine the required number of bytes to read from the socket buffer without altering the decoder's internal state. Makes reader cancellation safe on application layer, but stream is cancellation unsafe.

[lucasbalieiro]

hey, still wrapping up the review here.

but with the changes in this PR, I managed to make the pool unkillable again, which brings back the zombie task issue we thought was fixed in #1764.

If you want to reproduce the zombie state, just follow the steps I shared in #1763 .