looking for hint to debug a strange multi-thread bug (grpc-d-core)

[mw]

UPDATE: life is too short to debug dlang built-in AA to right, 
let's just use HashMap from emsi_containers

https://github.com/mw66/grpc-d/blob/master/source/grpc/server/package.d#L25
```
HashMap!(string, ServiceHandlerInterface) services;
```

After this change, the problem goes away.

I think there are some bugs in the builtin AA implementation, as 
demo-ed in this multi-threaded program (maybe also related to 
GC?).