Refactor memory strategies

[wlandau]

Prework

• Read and abide by drake's code of conduct.
• Search for duplicates among the existing issues, both open and closed.

Description

I propose a couple new/changed memory strategies.

memory_strategy	Initial discard	Initial load	Final discard
"preclean"	Discard all targets which are not dependencies of the current target.	Load any missing dependencies.	Keep the return value loaded.
"autoclean"	Discard all targets which are not dependencies of the current target.	Load any missing dependencies.	Unload the return value.

The "preclean" memory strategy is currently called "memory", which is not a very good name.