Keep capacity when dropping pending reads in the stream channel

[glbrntt]

Motivation:

When the stream channel has closed it drops all of its pending reads. It does this by calling 'removeAll()' on its CircularBuffer. The buffers underlying storage is emptied and then a nil is appended (internally CircularBuffer is never empty) which triggers an allocation.

Modifications:

• Remove all but keep capacity

Result:

Fewer allocations