Refactor/network improvements

Summary

• Rename trait rpc::Request to RequestArgs to be more specific and to reduce the number of structs named Request
• Add network::Request (private) struct instead of an unnamed tuple for the request queue in the ConnectionPoolWorker
• Remove ConnectionPoolBuilder struct to reduce code complexity. Builders should also be added in cases their use is a benefit, i.e. large numbers of parameters, sequences of parameters, etc. This is not the case with ConnectionPool
• Make connection pool's methods take &self by hiding the mutability internally using the UnsafeCell. This is safe, because 2 raw pointers into the UnsafeCell will never exist simultaneously, unless we use ConnectionPool from separate threads, which we aren't going to.
• Refactor ConnectionPool's code to reduce copy-paste
• Add ability to specify a timeout per request. If the timeout is not specified, the one from the WorkerOptions is used.
• Reuse the single ConnectionPool between raft node loop and governor loop

Ensure that

• New code is covered by tests
• API is documented
• Changelog is up to date
• (if Lua API changed) Lua API version is bumped in luamod.rs
• (if API docs changed) A follow-up doc issue is created in picodata/docs and linked here