libbinder: dynamically accept clients

Server listens on a single port and add clients.

The server looks like this:

    while True:
        accept client
        read client id
        if new id:
            create new rpc connection
        else:
            attach thread to existing rpc connection

Roadmap:
- having client add connections only when needed (currently they are
  all added at initialization time) - when this change is made, the
  server will also need to enforce the max threads per client.
- allowing RpcConnection to create reverse connections with an
  threadpool to serve calls in the other direction
- replacing connection IDs with something like TLS
- access controls for who can connect to who in pKVM context

Bug: 185167543
Test: binderRpcTest
Change-Id: I510d23a50cf839c39bc8107c1b0dae24dee3bc7b