memOverheadChangedCallback heap-use-after-free during ~VBucket

The memOverheadChangedCallback accesses the VBucket state.
During VBucket destruction the callback is triggered when destroying the CheckpointManager as this in turn destroys Checkpoint s, reducing overhead. The state is no longer valid at this point.

First seen in CV from merging patches introducing the memOverheadChangedCallback forward to master. Not previously identified on mad-hatter, possibly as a result of mad-hatter CV using an older clang.

http://cv.jenkins.couchbase.com/job/kv_engine.threadsanitizer/job/master/7760/consoleFull#-130641249961882284-c5b1-40af-8076-4f8cb2d12fb1