Create worker_proto::{Read,Write}Conn

Pass this around instead of `Source &` and `Sink &` directly. This will
give us something to put the protocol version on once the time comes.

To do this ergonomically, we need to expose `RemoteStore::Connection`,
so do that too. Give it some more API docs while we are at it.

src/libstore/remote-store-connection.hh

@@ -0,0 +1,97 @@
+#include "remote-store.hh"
+#include "worker-protocol.hh"
+
+namespace nix {
+
+/**
+ * Bidirectional connection (send and receive) used by the Remote Store
+ * implementation.
+ *
+ * Contains `Source` and `Sink` for actual communication, along with
+ * other information learned when negotiating the connection.
+ */
+struct RemoteStore::Connection
+{
+    /**
+     * Send with this.
+     */
+    FdSink to;
+
+    /**
+     * Receive with this.
+     */
+    FdSource from;
+
+    /**
+     * Worker protocol version used for the connection.
+     *
+     * Despite its name, I think it is actually the maximum version both
+     * sides support. (If the maximum doesn't exist, we would fail to
+     * establish a connection and produce a value of this type.)
+     */
+    unsigned int daemonVersion;
+
+    /**
+     * Whether the remote side trusts us or not.
+     *
+     * 3 values: "yes", "no", or `std::nullopt` for "unknown".
+     *
+     * Note that the "remote side" might not be just the end daemon, but
+     * also an intermediary forwarder that can make its own trusting
+     * decisions. This would be the intersection of all their trust
+     * decisions, since it takes only one link in the chain to start
+     * denying operations.
+     */
+    std::optional<TrustedFlag> remoteTrustsUs;
+
+    /**
+     * The version of the Nix daemon that is processing our requests.
+     *
+     * Do note, it may or may not communicating with another daemon,
+     * rather than being an "end" `LocalStore` or similar.
+     */
+    std::optional<std::string> daemonNixVersion;
+
+    /**
+     * Time this connection was established.
+     */
+    std::chrono::time_point<std::chrono::steady_clock> startTime;
+
+    /**
+     * Coercion to `WorkerProto::ReadConn`. This makes it easy to use the
+     * factored out worker protocol searlizers with a
+     * `RemoteStore::Connection`.
+     *
+     * The worker protocol connection types are unidirectional, unlike
+     * this type.
+     */
+    operator WorkerProto::ReadConn ()
+    {
+        return WorkerProto::ReadConn {
+            .from = from,
+        };
+    }
+
+    /**
+     * Coercion to `WorkerProto::WriteConn`. This makes it easy to use the
+     * factored out worker protocol searlizers with a
+     * `RemoteStore::Connection`.
+     *
+     * The worker protocol connection types are unidirectional, unlike
+     * this type.
+     */
+    operator WorkerProto::WriteConn ()
+    {
+        return WorkerProto::WriteConn {
+            .to = to,
+        };
+    }
+
+    virtual ~Connection();
+
+    virtual void closeWrite() = 0;
+
+    std::exception_ptr processStderr(Sink * sink = 0, Source * source = 0, bool flush = true);
+};
+
+}