QRingBuffer: cache the last released block

A typical ring buffer usage is a sequence of reserve()->chop()->read()
cycles. Usually, between these cycles, the buffer doesn't contain data and
all blocks are released. To reduce reallocations, keep the most recently
used block while the buffer is empty.

Change-Id: I8128f1f04649ae005fd0a480f17f95de01a9a135
Reviewed-by: Oswald Buddenhagen <oswald.buddenhagen@theqtcompany.com>