Nuclide HHVM Debugger - Switch to synthetic thread IDs - 12/N

Nuclide HHVM Debugger - Switch to synthetic thread IDs - 12/N

Summary:
This diff switches the IDs we present the front end for threads from the actual process thread IDs in HHVM to an artificial int assigned by the debugger engine.

There are a few reasons for needing to do this:

1. Actual thread IDs are uint64_t and the protocol calls for a JavaScript Number, which has a much lower max value.
2. The huge numbers look really strange in the debugger client, they are user visible as "Request ID"
3. This reveals internal implementation details about how HHVM is executing requests that are not relevant / not the user's business
4. Request threads are handled by a thread pool. This means the same request ID is seen exiting and then starting again repeatidly in the UX. This would appear strange to an astutue user, and while Nuclide doesn't care, I'd expect some implementors of the VS Code protocol might trip on this.
5. The logic in client attach that installs the hook on any requests that are already running was actually buggy - it was calling Process::GetThreadID in attachToRequest, which was running on the client connect thread, not the actual request thread.

Reviewed By: rightparen

Differential Revision: D6336882

fbshipit-source-id: 2cf7647b5f91e23f3561bee7a95c2352542f656b