Fixes #3803

New config setting for tracker:

; The window to look back for a previous visit by this current visitor. Defaults to visit_standard_length.
; If you are looking for higher accuracy of "returning visitors" metrics, you may set this value to 86400 or more.
; This is especially useful when you use the Tracking API where tracking Returning Visitors often depends on this setting.
; The value window_look_back_for_visitor is used only if it is set to greater than visit_standard_length
window_look_back_for_visitor = 0