Add special case for TargetLogLatency == 0

A bit of a rough hack, but just avoid the ticker goroutine altogether.
An alternative would be to tune the RequestSizeTrigger to zero, but as
it this is an exposed field it seems changing that would cause
astonishment among consumers when the value is zero regardless of the
initial setting because of a target latency of zero.

Signed-off-by: Daniel Farina <daniel@fdr.io>