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/local-store.hh

@@ -121,6 +121,8 @@ public:
 
     StringSet queryDerivationOutputNames(const Path & path);
     
+    Path queryPathFromHashPart(const string & hashPart);
+    
     PathSet querySubstitutablePaths();
     
     bool hasSubstitutes(const Path & path);
@@ -217,6 +219,7 @@ private:
     SQLiteStmt stmtAddDerivationOutput;
     SQLiteStmt stmtQueryValidDerivers;
     SQLiteStmt stmtQueryDerivationOutputs;
+    SQLiteStmt stmtQueryPathFromHashPart;
 
     int getSchema();