Fixed Issues in Apache Kudu

Review the list of Kudu issues that are resolved in Cloudera Runtime 7.1.6.

KUDU-1954: Improve maintenance manager behavior in heavy write workload
Fixed lock contention between MaintenanceManager’s threads performing already scheduled operations and the scheduler thread itself. This benefits clusters with heavy ingest/update workloads that have many replicas per tablet server.
KUDU-3108: Tablet server crashes when handle diffscan request
Fixed a bug in the merge iterator that could result in a crash. This could surface as a crash when performing ordered or differential scans, particularly when the underlying data contained deletes and reinserts.
KUDU-3149: Lock contention between registering ops and computing maintenance op stats
Fixed lock contention between MaintenanceManager op registration and the scheduling of new maintenance ops. On particularly dense tablet servers, this contention was previously shown to significantly slow down startup times.
KUDU-3205: NPE in KuduScanTokenBuilder#build after a tablet server goes down
Fixed a bug where building scan tokens would result in a NullPointerException if a tablet not found error occurred before generating the token.
KUDU-3238: Use-after-free condition in Kudu C++ client upon modifying a table's partitioning
Fixed a heap-use-after-free bug in Kudu C++ client that might manifest itself when altering a table to update the partitioning schema.