C++-ize inproc transport

### Should this be an issue in the gRPC issue tracker?

Yes

### What version of gRPC and what language are you using?

Core, C++

### What operating system (Linux, Windows, …) and version?

N/A

### What runtime / compiler are you using (e.g. python version or version of gcc)

N/A

### What did you do?

Read the code for in-process transport

### What did you expect to see?

Clean idiomatic C++ code

### What did you see instead?

C-style code

### Anything else we should know about your project / environment?