Select of Blink Table should be Flat

[cpwright]

As a user, I want select on a blink table to produce a flat table so that the select will have memory usage bounded by the largest update rather than the number of row keys that it sees over time.

[nbauernfeind]

Maybe we should create a blink-table supported version of flatten which redirects column sources using the added rowset. We could invoke the flatten on the source table and then defer the select on that sub-table. Same idea, but maybe provides a better encapsulation of logic.

[cpwright]

If you flatten before select, it should have the same effect; and select() could throw that into the preamble in io.deephaven.engine.table.impl.QueryTable#selectInternal that already flattens for many static selects. The flatten should convert all adds/removes to adds and removes from 0...n-1.

From a user-perspective, however, we should ideally never have to think about our address space and flattening it, when the engine can do it for us without loss of performance.

[cpwright]

I said that it was equivalent, the result is equivalent; but the performance is a little worse as the select operation would need to read through a redirection instead of just writing out to flat (but still growable) sources.

[rcaudy]

Do we want the same flattening for append-only? I think yes.