Backpressure is missing

[JakeWharton]

The preference observables do not handle backpressure which will lead to an exception when it's applied aggressively.

Handling backpressure in a system like this is interesting because it should be applied after the filter operator rather than inside of the OnSubscribe. This ensures the pressure is applied to potentially-emitted stream values rather than simply any change to any key. When backpressure is applied and exceeded, values should be dropped and not emitted downstream (like onBackpressureDrop()). When backpressure is let off (i.e., more values are requested) and values were dropped, the operator should immediately emit a single value (the value of which doesn't matter because it's unused) so that the latest preference value is loaded and sent out.

[JakeWharton]

Oh, snap. I already wrote this: https://github.com/square/sqlbrite/blob/1479605b02a0e63344dfbb393c762e75233eeec1/sqlbrite/src/main/java/com/squareup/sqlbrite/BackpressureBufferLastOperator.java