Expose READ_ONLY_MODE_KEY property for DatabaseFileSource

[pozdnyakov]

This PR exposes DatabaseFileSource property, which allows to re-open the offline database in read-only mode and thus improves the performance for the set ups that do not require the offline database modifications. Example of usage:

auto fileSource = mbgl::FileSourceManager::get()->getFileSource(
    mbgl::FileSourceType::Database, resourceOptions);
fileSource->setProperty(READ_ONLY_MODE_KEY, true);

Tag https://github.com/mapbox/mapbox-gl-native-team/issues/169

[alexshalamov]

lgtm, would be nice to create issue for setProperty that would have optional callback to indicate that setter has failed.