Separate TA caching logic from RPP caching logic

Separate TA caching logic from RPP caching logic

(Was getting shot in the foot by overly reusing code.)

Testing and course correcting, week 2. Unstable.

Fixes (from the previous commit)

> 2. Fall back to rsync only on RRDP download failure; it's wasteful to
>    also do it on RRDP validation failure.

RPPs:
	RRDP download failure triggers rsync download fallback.
	RPP validation failure no longer triggers rsync download
	fallback.

TAs:
	HTTP download failure triggers rsync download fallback.
	HTTP validation failure still triggers rsync download fallback.
	(Because it's the TA, and needs to be treated more
	paranoically.)

This commit is also a startup for

> 3. Must commit repository content at the RPP level, not at the module
>    level. (One failing RPP should not invalidate the whole module.)

Because I'm headed into a more explicit separation between "Repository"
and "RPP."