Change middleware hooks to be wrappers for Mongo driver methods

[ericyhwang]

We're finding more and more that it can be useful to intercept Mongo driver method calls in a way that also has sharedb / shareedb-mongo context, e.g.:
#120

The current approach is to have middleware hooks based on sharedb-mongo actions, e.g. Actions.beforeSnapshotLookup.

@alecgibson and I are thinking that it could be more flexible to have a generic hook based on the Mongo driver method name, e.g. find, that provides the sharedb-specific actions or contexts behind the call, and allows customizing all arguments passed to the driver.

That would require us to drop support for Mongo driver 2.x, as find only started taking an options object in Mongo driver 3.x.

• Driver 2.x find - https://mongodb.github.io/node-mongodb-native/2.2/api/Collection.html#find
• Driver 3.x find - https://mongodb.github.io/node-mongodb-native/3.6/api/Collection.html#find
• Driver 4.x find - https://mongodb.github.io/node-mongodb-native/4.0/classes/collection.html#find

Mongo driver 2.x is no longer officially supported anyways. Also, the 2.x driver is only officially compatible up through Node 8, which was end-of-life as of 2019-12-31:
https://docs.mongodb.com/drivers/node/compatibility/