Add cross-process pipes, ensure that they aren't free'd

[mrmr1993]

This PR builds upon #1338. This does several things:

• splits the reading and writing ends of pipes between the client (VM interpreter) and server processes
• keeps around references to all of the pipes, so that they don't get drop-ed and cause a reference to a freed file descriptor
• passes the correct ends of pipes to the processes, so that the corresponding read/write calls don't immediately fail.

This was tested by sending a dummy hint to the server process and logging a message to show it being received. Also, this now does not immediately crash.