Improve user control over camera connect/disconnect

[PeterJohnson]

Due to the sometimes long delay incurred during connecting a USB camera, the common desire to switch between cameras, and the problems of USB camera bandwidth limitations, more control over camera connection and disconnection is desirable, particularly for USB cameras. Currently the library auto-disconnects when no sinks are listening, necessitating hacky methods for keeping the camera connection open to avoid reconnect delays when there is a sink being switched between two sources. Also, if two cameras combined exceed the USB camera bandwidth, there is no way to synchronize one stopping and the other starting when setSource is used to switch.

It seems like the cleanest approach would be to make this auto-disconnection feature optional and probably default it to disabled (at least for USB cameras) and provide API access to explicitly connect/disconnect the camera.

[PeterJohnson]

Fixed by wpilibsuite/allwpilib#1252.