[dconf] read locked keys and skip them during gconf to dconf migration.

If we don't skip them then the we fail to save the DConf changeset
make sure the path exists before we construct a std::string out of it
log the exception if we fail to read locked keys for a path
Merge pull request #1 from foolab/master

[dconf] read locked keys and skip them during gconf to dconf migration.