Stop fold when a vnode reaches page size

Currently, 2i pagination may, with a sufficiently slow participating
node, receive many, many more keys than the page size (max_results) from
all the other participating nodes.
This change will track how many results have been received per vnode,
and send a stop fold message to a vnode as soon as it has sent more than
max_results.
Note, however, that vnodes will send in chunks possibly much larger than
the page size (100 items at a time I believe). It would be ideal to pass
max_results to the folding function and have it stop when it reaches
that number. But that will have to wait until later. Release is near.
This should at least prevent a worst case of unbounded growth that could
consume all the node memory
Merge pull request #586 from basho/eas-2i-pag-per-vnode-limit

Stop fold when a vnode reaches page size