plugin-manager: allow new ports up to 1500ms since last port added
Until now we had only a 2500ms timeout initialized since the first port was exposed until we decided we were ready to consider all ports notified by the kernel.
With this new logic, we add an additional condition: even if the 2500ms initial timeout has elapsed already, we leave an additional 1500ms since the last port addition for new ports to appear.
This new logic is useful when relying on the ReportKernelEvent() DBus method, as it is the user the one responsible for reporting the kernel events instead of udev. Now, the user is not forced to make sure all ports are exposed in 2500ms; instead, we also allow ports to be reported in more than 2500ms as long as the time between port additions reported is less than 1500ms.
Note that this does not mean that the whole probing time will now always be 4000ms. On well behaved systems (like when based on udev) this new 'extra' probing timeout may expire long before the 'min' probing timeout we already had as well.
E.g. in this setup, the reporting of the NET port was done 1100ms later than the last ttyUSB3, and that was already too late as the original 2500ms threshold had already expired.
[1573536994.593874] (tty/ttyUSB0): first port in device /sys/devices/platform/ehci-platform/usb1/1-1
[1573536994.596659] [plugin manager] task 1: port grabbed: ttyUSB0
[1573536995.093579] (tty/ttyUSB1): additional port in device /sys/devices/platform/ehci-platform/usb1/1-1
[1573536995.094172] [plugin manager] task 1: port grabbed: ttyUSB1
[1573536995.603206] (tty/ttyUSB2): additional port in device /sys/devices/platform/ehci-platform/usb1/1-1
[1573536995.603822] [plugin manager] task 1: port grabbed: ttyUSB2
[1573536996.111564] (tty/ttyUSB3): additional port in device /sys/devices/platform/ehci-platform/usb1/1-1
[1573536996.112257] [plugin manager] task 1: port grabbed: ttyUSB3
[1573536996.814816] [device /sys/devices/platform/ehci-platform/usb1/1-1] creating modem with plugin 'Quectel' and '4' ports
[1573536997.265820] (net/wwan0): additional port in device /sys/devices/platform/ehci-platform/usb1/1-1
[1573536997.296935] (usbmisc/cdc-wdm0): additional port in device /sys/devices/platform/ehci-platform/usb1/1-1