Improve the cache fallback algorithm

Improve the cache fallback algorithm

If all of a RPP's URLs fail, fall back to most sensible cached
candidate.
It seems this used to be only implemented for TAs, and the heuristics
for choosing a suitable fallback were rudimentary.
Elaborate, centralize and extend implementation to all cache content.

Side maintenance tweaks:

- Remove EREQFAILED, because it largely evolved from meaning "don't try
  again" to "try again." So now it was just a redundant EAGAIN.
- Ditch redundant arguments from valid_file_or_dir().
- Merge the three URI arraylist implementations (certificate.c, tal.c
  and manifest.c) into one.
- Move RRDP workspace URI, from a thread variable to the stack.
  (Code smell. It used to be awkward to follow this variable's lifespan
  through the tree traversal.)
- Move struct publish and struct withdraw from the heap to the stack.
  (Eliminate pointless allocations. These are not the only RRDP
  structures I want to move to the stack.)
- Change file_metadata.uri from `char *` to `struct rpki_uri *`.
  (This string was forcing the RRDP code to recompute the URI
  repeatedly.)