v0.5.2

• Support to build in-memory index with multiple threads to shorten db open time
• Added new option to set number of threads used to build index, default is 1
• Added new option to set tombstone file size separately. It will fall back to use data file size by default
• Provide API to clients to merge tombstone files on demand instead of doing that during db open
• Expose tombstone and data file count to stats for monitoring purpose
• Provide API to covert stats to string map for convenience