Port AccessScanner to pause/resume visitor

At the moment it builds a list (set) of all keys in a vbucket, then writes to disk. This is so we don't perform disk in the vbucket callback.

Problem with this is that we need a massive chunk of memory to hold all the keys.

Better approach would be to use the pause/resume visitor to say visit N keys, write to the disk, then continue. Also means we don't hold HashTable locks for long times (AccessScanner doesn't have to include every key, given it may well change in 1 days' time).