Do we want "aquarium-direct-map" any more?

[hujiajie]

I have mixed feeling about dropping //src/aquarium-direct-map.

Pros

• Our maintenance effort can be reduced.

Cons

• It can work as a perf baseline to tell whether abstraction in the "optimized" variant is over-engineered.
• If we prefer to remove it, further discussion about the new directory hierarchy is needed. For example, //src/aquarium-optimized/* can be moved to //src/*, and //src/common seems no longer necessary as well.
• The overall maintenance effort probably won't be reduced much. Usually the difficult part is to come up with a design out of thin air, instead of mirroring the change from one directory to the other.

[hujiajie]

The decision is to remove aquarium-direct-map.