atexit won't run registered functions for SIGTERM which we send to the server.
This prevents clean shutdown.
Also making sure that the server logfiles are deleted as well.
Previously the YCM Vim client would go bonkers when ycmd crashed. Now the user
can continue using Vim just without YCM functionality.
Also added a :YcmRestartServer command to let the user restart ycmd if it
crashed. With a little luck, this will be rarely necessary.
This means we can now load just ycm_client_support (which is a much smaller
library) into Vim and ycm_core into ycmd. Since ycm_client_support never depends
on libclang.so, we never have to load that into Vim which makes things much,
much easier.
