plfs: defer data/index dropping creation

In open for write, don't create empty data and index files.
Only create them in the first write. Hostdir creation is also
deferred to the first write when possible.

A configurable is added in plfsrc to control the logic.
Add "lazy_droppings 0" to plfsrc to disable the feature.
Merge pull request #177 from bergwolf/container_empty_files_rebased

plfs: defer data/index dropping creation