Perforce polling uses up too much CPU

Make perforce polling more efficient, to reduce CPU usage.

Currently we need to make 3-4 calls to the perforce server every time we poll it for changes. These calls are relatively CPU intensive (text parsing). We need to look at finding a way to either reduce the number of calls required or to make them a bit more efficient.

One idea is to let the user input information that we regularly retrieve for example the client root or the left hand side of the depot mapping to allow the user to make the decision between maintainability (harder to keep bamboo and perforce in sync) and performance.