* Eliminate all uses of the global variable ‘store’ from libstore.

This should also fix:

    nix-instantiate: ./../boost/shared_ptr.hpp:254: T* boost::shared_ptr<T>::operator->() const [with T = nix::StoreAPI]: Assertion `px != 0' failed.

  which was caused by hashDerivationModulo() calling the ‘store’
  object (during store upgrades) before openStore() assigned it.

src/libstore/misc.hh

@@ -9,7 +9,7 @@ namespace nix {
 
 /* Read a derivation, after ensuring its existence through
    ensurePath(). */
-Derivation derivationFromPath(const Path & drvPath);
+Derivation derivationFromPath(StoreAPI & store, const Path & drvPath);
 
 /* Place in `paths' the set of all store paths in the file system
    closure of `storePath'; that is, all paths than can be directly or
@@ -18,7 +18,7 @@ Derivation derivationFromPath(const Path & drvPath);
    `storePath' is returned; that is, the closures under the
    `referrers' relation instead of the `references' relation is
    returned. */
-void computeFSClosure(const Path & storePath,
+void computeFSClosure(StoreAPI & store, const Path & storePath,
     PathSet & paths, bool flipDirection = false,
     bool includeOutputs = false);
 
@@ -29,7 +29,7 @@ Path findOutput(const Derivation & drv, string id);
 /* Given a set of paths that are to be built, return the set of
    derivations that will be built, and the set of output paths that
    will be substituted. */
-void queryMissing(const PathSet & targets,
+void queryMissing(StoreAPI & store, const PathSet & targets,
     PathSet & willBuild, PathSet & willSubstitute, PathSet & unknown,
     unsigned long long & downloadSize, unsigned long long & narSize);