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 + RwLock has been replaced by thread_local!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.
Default features of config have also been disabled, removing 32 unused crate dependencies from the final build.
Note that the write_setting currently overrides the previous configuration rather than merging it (hence, the fix name before the commit name).
Edited by Vincent Lafeychine
