racetest: Refactor tests to work via combining testing model with checkers

Introduce the notion of testing model to represent initial state,
eveolution step and maintained invariant in the database.

Refactor tests to accept arbitrary such model and to drive them through
particular testing scenarious that try to hit specific concurrency
issues.

Not only this deduplicates the code a bit, but it also will allow us to
run different models through existing and future checkers. Soon we will
need to run a model with 3 variables (obj1, obj2 and phase) instead of
only 2 (obj1 and obj2) for invalidate / disconnect test.

Please see added module-level documentation for overview.