Improve SSH handling

* Unify SSH code in SSHStore and LegacySSHStore.

* Fix a race starting the SSH master. We now wait synchronously for
  the SSH master to finish starting. This prevents the SSH clients
  from starting their own connections.

* Don't use a master if max-connections == 1.

* Add a "max-connections" store parameter.

* Add a "compress" store parameter.

src/libstore/ssh.hh

@@ -0,0 +1,41 @@
+#pragma once
+
+#include "util.hh"
+
+namespace nix {
+
+class SSHMaster
+{
+private:
+
+    std::string host;
+    std::string keyFile;
+    bool useMaster;
+    bool compress;
+    Pid sshMaster;
+    std::unique_ptr<AutoDelete> tmpDir;
+    Path socketPath;
+
+public:
+
+    SSHMaster(const std::string & host, const std::string & keyFile, bool useMaster, bool compress)
+        : host(host)
+        , keyFile(keyFile)
+        , useMaster(useMaster)
+        , compress(compress)
+    {
+    }
+
+    struct Connection
+    {
+        Pid sshPid;
+        AutoCloseFD out, in;
+    };
+
+    std::unique_ptr<Connection> startCommand(const std::string & command);
+
+    void startMaster();
+
+};
+
+}