Implement memory-based bookkeeping

[muvaf]

What problem are you facing?

See #36 for details.

How could Terrajet help solve your problem?

While some of the ideas do not require memory-based bookkeeping, it's a solution to the problem where we need to make the same call to retrieve its results. In other words, we could make it work with filesystem locks but I don't believe we need to bear the cost of that complexity if we can do it in memory in a self-updating way.

The main point of this issue that the layer that ExternalClient implementation will interact with should look very similar to a contemporary cloud SDK API so that we don't break assumptions of generic reconciler. This includes simplifying the inner workings of that SDK as well by reducing the number of layers there which we added to be able to work in parallel during the initial phase.