mongodb-schema should take a memory limit parameter and stop analysing the schema when that limit is reached.

It could either (a) stop adding values and simply update counts/probabilities, or (b) stop sampling documents alltogether.

It should probably also contain a top-level field "memoryLimitReached: true", so that Compass can give the user some feedback.

This work has already been started as part of COMPASS-218 but is too risky to merge for 1.5 stable. Re-visit after 1.5 was released.