paging support for annotation count query

[tingzhao]

Counting annotations in labelsz can take a long time when the data are big, potentially causing timeout in the client side (e.g. 10.40.4.158:8400/api/node/65c0/synapses_labelsz/top/5000/AllSyn). Paging can be a solution to this problem.

[DocSavage]

Did you have an idea how you’d like the paging solution to work or should I just come up with the first pass and we’ll see if it works for you?

[tingzhao]

The page option for threshold looks fine to me, or whatever you think is efficient.