Add lock to cfunction.go
Add lock to cfunction.go

cfunction.go is one of the places that we maintain some global state,
so we need to add a lock to protect from concurrent modification.

limetext/gopy,limetext/gopy,ericsnowcurrently/qur-gopy,qur/gopy,qur/gopy,ericsnowcurrently/qur-gopy,limetext/gopy,qur/gopy,ericsnowcurrently/qur-gopy