Make it possible for FileStore to be in a state which never do merge and keep it backward compatible

Make the configuration "filestore merge threshold" can be negative which prevent it from merging, this could help:

  1. We are trying to create the PG folder up to several levels with a standalone tool to prevent it from runtime splitting, we need a configuration which prevent it from merging even there is no file within the folder.
  2. As runtime split / merge could bring latency issues, customer can use a negative merge threshold to prevent merging but only splitting.
This change is backward compatible.
Signed-off-by: Guang Yang (yguang@yahoo-inc.com)