Data rotation in the read buffer creates latency, we know it
is preset but so far its magnitude was unknown. This PR adds
it as a new field to the usage struct. For example, the
test_conn_echo_stress outputs now
Commands sent: 780,002
Bytes sent: 32,670,085
Responses received: 780,001
Pushes received: 750,001
Bytes received (response): 3,210,147
Bytes received (push): 32,250,036
Bytes rotated: 3,109,190,184
In total approximately 34Mb are received but 3Gb are
rotated.
a70bdf6574