Release v0.5.7

New functions:

• Add r/w SLA info in SDK
• Load check can ignore file-system error of parent-tablets' directory
• Support update schema of old-style kv-table
• Remove first query RPC for new tabletserver to speed up recovery
• Add timeout precision flags into SDK
• Tabletserver unloads all tablets on exit

Bugfix:

• delay task may continue to run after Table object deleted by user