Factor out a LogStore interface

Continue progress on #5729.

Just as I hoped, this uncovered an issue: the daemon protocol is missing
a way to query build logs. This doesn't effect `unix://`, but does
effect `ssh://`. A FIXME is left for this, so we come back to it later.

src/libstore/store-api.hh

@@ -605,14 +605,6 @@ public:
      */
     StorePathSet exportReferences(const StorePathSet & storePaths, const StorePathSet & inputPaths);
 
-    /* Return the build log of the specified store path, if available,
-       or null otherwise. */
-    virtual std::optional<std::string> getBuildLog(const StorePath & path)
-    { return std::nullopt; }
-
-    virtual void addBuildLog(const StorePath & path, std::string_view log)
-    { unsupported("addBuildLog"); }
-
     /* Hack to allow long-running processes like hydra-queue-runner to
        occasionally flush their path info cache. */
     void clearPathInfoCache()