Add recent changes and fixes for shutdown bugs.
https://github.com/chaincodelabs/libmultiprocess/pull/111: doc: Add internal design section https://github.com/chaincodelabs/libmultiprocess/pull/113: Add missing include to util.h https://github.com/chaincodelabs/libmultiprocess/pull/116: shutdown bugfix: destroy RPC system before running cleanup callbacks https://github.com/chaincodelabs/libmultiprocess/pull/118: shutdown bugfix: Prevent segfault in server if connection is broken during long function call https://github.com/chaincodelabs/libmultiprocess/pull/119: cmake: avoid libatomic not found error on debian