Consider refactor to cast between snapshot and doc like sharedb-mongo

Filing @nateps's comment from a PR to keep track of it:

At first glance, I think it might be better to cast the format in which we store the docs rather than cast the query. This is how sharedb-mongo works, so I think it would be easier to make it more consistent. We might also be able to share code by creating another module that this and sharedb-mongo depend on or by at least copy-pasting and cross referencing the code.

Switching to snapshot/doc casting may make it easier to have filtering be fully consistent with sharedb-mongo.

In sharedb-mongo, query filters on special Share properties look like {'_v': {$lte: 10}}. https://github.com/share/sharedb-mingo-memory/pull/3 added sharedb-mingo-memory support for filtering on all such special properties except Share metadata. An example of such a metadata query is {'_m.mtime': {$gte: 100}}. I added a currently-skipped test for that.

Start by reading the skipped metadata-query test in test/query.js and the referenced sharedb-mongo implementation, then review PR #3 to understand the current casting approach. The refactor should make snapshot/doc casting consistent with sharedb-mongo and enable the _m.mtime query test to pass.