plfs: better container deletion

Defer container deletion until last moment so that if anything fails in the
middle, the container informantion remains.

Change-Id: I035d892b6e344ba5faa94b2184c5327153e84128
Implement small file mode for PLFS.

Small file aggregation mode can enable aggregation on both data
and metadata which can greatly improve the performance for small
IOs such as creating lots of files and appending data to them.
Merge pull request #1 from bergwolf/container_delete_for_qa

plfs: better container deletion
Merge pull request #169 from zhang-jingwang/small_file

Small file