All partitions will have a corresponding mutation object in the buffer.
These objects have non-negligible sizes, yet the consumer did not bump
the _buffer_size when a new partition was consumer. This resulted in
empty partitions not moving the _buffer_size at all, and thus they could
accumulate without bounds in the buffer, never triggering a flush just
by themselves. We have recently seen this causing OOM.
This patch fixes that by bumping the _buffer_size with the size of the
freshly created mutation object.
d66b07823b