OVSDriver: use a socket to send kflow requests to the main thread

Reviewer: @harshsin 

IVS is moving to a multi-process model, so we need to use IPC instead of a
linked list to communicate with the main thread. The advantage of a datagram
socket over a pipe is that the kernel preserves message boundaries.

Each upcall thread now gets its own queue, which improves fairness over the
previous implementation.

The bh submodule is now unused so I've deleted it.