Server changes:

- for each VM and template function, indicates whether the function is "concurrency-safe" (that is: whether the function can be called concurrently by multiple threads and still behave correctly)
- simplified many portions of the codebase (notable vm.go and vms.go)
- went through the whole template and VM code and updated it to ensure there are no concurrency bugs (well, as best as I could)
- documented missing parts of the codebase
- fixed a few bugs in the way DiskBusy was handled in VM/template code
- bumped version to 1.8.5