[census builder] improve state isolation in the validator sub-system

[bkmartinjr]

The census builder validator is not sufficiently isolated from the rest of the builder. This makes it more complex than necessary (e.g., reopen of experiments), and leaves it open to possible state contamination from the build process

I recommend:

• the validator should be driven by a subset of the ExperimentBuilder object which minimizes exposure to bugs in the build process itself. For example, the experiment handle should be off limits, as should build-time accumulators (eg., n_obs).
• the validator should not re-use the SOMA/TileDB context, and should instead use a clean/new context, with default timestamp, so as to mimic reading behavior of (most) other users.

See discussion here.

See related #179