Add function queryPathFromHashPart()

To implement binary caches efficiently, Hydra needs to be able to map
the hash part of a store path (e.g. "gbg...zr7") to the full store
path (e.g. "/nix/store/gbg...kzr7-subversion-1.7.5").  (The binary
cache mechanism uses hash parts as a key for looking up store paths to
ensure privacy.)  However, doing a search in the Nix store for
/nix/store/<hash>* is expensive since it requires reading the entire
directory.  queryPathFromHashPart() prevents this by doing a cheap
database lookup.

src/libstore/store-api.hh

@@ -139,6 +139,10 @@ public:
 
     /* Query the output names of the derivation denoted by `path'. */
     virtual StringSet queryDerivationOutputNames(const Path & path) = 0;
+
+    /* Query the full store path given the hash part of a valid store
+       path, or "" if the path doesn't exist. */
+    virtual Path queryPathFromHashPart(const string & hashPart) = 0;
     
     /* Query whether a path has substitutes. */
     virtual bool hasSubstitutes(const Path & path) = 0;