[pantsd] Save a copy of the pantsd logs on clean-all and kill-pantsd

[blorente]

Problem
When diagnosing pantsd issues, it's really hard to get accurate pantsd logs, because the first thing the user tries is ./pants clean-all, which removes anything of utility.

Solution
Store a pid-indexed copy of the pantsd logs either:

• Whenever the daemon restarts, so on PantsDaemon.terminate().
• Whenever we run a goal that would kill pantsd (./pants clean-all and ./pants kill-pantsd).

Result
Whenever we shutdown pantsd, a file like .pants.d/pantsd/pantsd.1234.log should be there after everything is said and done.

[blorente]

Closing this as a duplicate of #8196