Write-ahead log garbage collection

Kudu maintains a write-ahead log (WAL) per tablet that is split into discrete fixed-size segments. A tablet periodically rolls the WAL to a new log segment when the active segment reaches a size threshold (configured by the --log_segment_size_mb property).

In order to save disk space and decrease startup time, a background task called LogGCOp attempts to garbage-collect (GC) old WAL segments by deleting them from disk once it is determined that they are no longer needed by the local node for durability.

The metrics associated with this background task have the prefix log_gc.