Start factoring out the serve protocol for Hydra to share

Factor out `ServeProto::BasicClientConnection` for Hydra to share

- `queryValidPaths`: Hydra uses the lock argument differently than Nix,
  so we un-hard-code it.

- `buildDerivationRequest`: Just the request half, as Hydra does some
  things between requesting and responding.

Co-authored-by: Robert Hensing <roberth@users.noreply.github.com>

src/libstore/serve-protocol-impl.cc

@@ -0,0 +1,38 @@
+#include "serve-protocol-impl.hh"
+#include "build-result.hh"
+#include "derivations.hh"
+
+namespace nix {
+
+StorePathSet ServeProto::BasicClientConnection::queryValidPaths(
+    const Store & store,
+    bool lock, const StorePathSet & paths,
+    SubstituteFlag maybeSubstitute)
+{
+    to
+        << ServeProto::Command::QueryValidPaths
+        << lock
+        << maybeSubstitute;
+    write(store, *this, paths);
+    to.flush();
+
+    return Serialise<StorePathSet>::read(store, *this);
+}
+
+
+void ServeProto::BasicClientConnection::putBuildDerivationRequest(
+    const Store & store,
+    const StorePath & drvPath, const BasicDerivation & drv,
+    const ServeProto::BuildOptions & options)
+{
+    to
+        << ServeProto::Command::BuildDerivation
+        << store.printStorePath(drvPath);
+    writeDerivation(to, store, drv);
+
+    ServeProto::write(store, *this, options);
+
+    to.flush();
+}
+
+}