This CL fixes the following problems
1. check whether TabletsChannel has been closed/cancelled in `reduce_mem_usage` to avoid using a closed DeltaWriter
2. make `FlushHandle.wait` wait for all submitted tasks to finish so that memtable is deallocated before its delta writer
3. make `~MemTracker()` release its consumption bytes to accommodate situations in aggregate_func.h that bitmap and hll call `MemTracker::consume` without corresponding `MemTracker::release`, which cause the consumption of root tracker never drops to zero