[CBM] Add a flag to take a heap profile periodically

We already have a developer flag to allow heap profiles to be taken. We should extend this to allow taking heap profiles repeatedly (with a specified delay) so that we can get an idea of how the memory usage over a run of cbbackupmgr changes.

In general the memory usage should be fairly flat but this could of course regress and sometimes we want to get a profile at around about the same time on two runs, without having to manually time things.