Replace deprecated methods in config

This MR replaces the deprecated Config::set method by using ConfigBuilder::add_source on the previously built configuration.

Also, since rustc is single-threaded, lazy_static! + RwLock has been replaced by thread_local! + RefCell.

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Default features of config have also been disabled, removing 32 unused crate dependencies from the final build.

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Note that the write_setting currently overrides the previous configuration rather than merging it (hence, the fix name before the commit name).