plfs: defer data/index dropping creation

Please pull deferring empty dropping creation patch to get following optimization. The code is based on the head of small_file branch.

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.