Spring cleaning

[hlinnaka]

Go through the repos and "git rm" obsolete stuff:

• Is contrib/zenith_store/memstore.* still needed? We have the Rust pageserver now.
• Which of the programs in src/bin/zenith_* are still needed?

Update all the READMEs

[kelvich]

Looks like that is generally done.

[hlinnaka]

Yeah. Although it's probably time for a new one already :-). We should be tidy continuously, really.