Petra AI - maximize population growth when having enough resources

[elexis]

When having enough resources (for example in games with deathmatch settings, or when having enough food in the bank for a batch of 10 women f.e.),

• houses have to be built as fast as possible so that
• as many units as possible can be trained simultaneously

More units can be tasked to build houses in that case as resources are abundant already and gathering more won't help in that situation.

[mimo]

In 20837:

Improve petra behavior in death-match: allow more builders when plenty of resources and replace tests on current population by tests on current + in-training-queue population (that allows faster phasing up).

Fixes #4168