CXXCBC-242: SDK Support for Native KV Range Scans

[DemetrisChr]

Updated the implementation to follow the newest RFC changes

Notable changes

• Added prefix_scan as top-level scan type
• Changed scan terms from byte vectors to strings
• Seed for sampling scan is randomly generated if not specified
• Removed range_scan_cancelled error code, returning request_canceled instead
• Removed sorting
• Added concurrency option to orchestrator to set the maximum allowed level of concurrency
• The constructor of range_scan_orchestrator takes the vbucket map instead of the number of vbuckets
• Implemented new concurrency approach for scanning vbuckets:
  • Streams start with the maximum level of concurrency. If a temporary failure occurs (i.e. receiving busy status from the server) the stream is retried a later point and the concurrency is reduced by 1
  • When a stream finishes successfully or with a benign error, another stream is initiated to take its place. If the temporary failure occurs, a new stream is not initiated which effectively reduces the concurrency (unless streams are not being executed concurrently in which case that continues)
  • The number of streams being run per node are kept track of and when a new stream should start, a vbucket on the least busy node is selected
• Renamed range_scan_timeout to key_value_scan_timeout in timeout defaults
• Removed batch_time_limit from options, 90% of the timeout is used instead
• Timeouts are now on both range_scan_create and range_scan_continue. There's also a check before retrying a stream if the time since the first attempt exceeds the timeout
• The next methods of the scan result can return an error code in the case of a fatal error
• Added cancel() method to scan_result that can cancel all streams
• Errors on range scan continue or start are now separated into 'fatal' (which takes into account whether the scan is a sampling scan), 'retryable' or 'benign'