[CBBS] Memory usage gradually increases during task

What is the problem?
Whilst looking at a customer case I spotted that CBBS memory usage (yellow) is gradually increasing during a long running task (cbbackupmgr blue).

Note the drop towards the end is a backup task finishing and then a new one starting. Looking at the code we collect stdout in a byte buffer that just continually grows

What is the solution?
We should limit the size of buffer in some way. At the moment we check this buffer periodically to get progress reports (pull). We could instead push updates from the runner to the bit of the code that gets progress reports. An alternative would be to use something like a ringbuf to limit the size of the buffer and read from this periodically