Treat unparseable (micro)descriptors and extrainfos as undownloadable

One pain point in evolving the Tor design and implementing has been
adding code that makes clients reject directory documents that they
previously would have accepted, if those descriptors actually exist.
When this happened, the clients would get the document, reject it,
and then decide to try downloading it again, ad infinitum.  This
problem becomes particularly obnoxious with authorities, since if
some authorities accept a descriptor that others don't, the ones
that don't accept it would go crazy trying to re-fetch it over and
over. (See for example ticket #9286.)

This patch tries to solve this problem by tracking, if a descriptor
isn't parseable, what its digest was, and whether it is invalid
because of some flaw that applies to the portion containing the
digest.  (This excludes RSA signature problems: RSA signatures
aren't included in the digest.  This means that a directory
authority can still put another directory authority into a loop by
mentioning a descriptor, and then serving that descriptor with an
invalid RSA signatures.  But that would also make the misbehaving
directory authority get DoSed by the server it's attacking, so it's
not much of an issue.)

We already have a mechanism to mark something undownloadable with
downloadstatus_mark_impossible(); we use that here for
microdescriptors, extrainfos, and router descriptors.

Unit tests to follow in another patch.

Closes ticket #11243.
Base tests for 11243: test parsing for md, ei, and ri.

We didn't really have test coverage for these parsing functions, so
I went and made some.  These tests also verify that the parsing
functions set the list of invalid digests correctly.
Unit tests for 11243: loading ri, ei, mds from lists

These tests make sure that entries are actually marked
undownloadable as appropriate.
Don't reset the download failure status of any object marked as impossible
Commit the script I used to generate signed ri and ei documents
Use symbolic constants for statuses in microdescs_add_to_cache.

Suggested by Andrea in her review of 11243.
Bugfixes on bug11243 fix for the not-added cases and tests

 1. The test that adds things to the cache needs to set the clock back so
    that the descriptors it adds are valid.

 2. We split ROUTER_NOT_NEW into ROUTER_TOO_OLD, so that we can
    distinguish "already had it" from "rejected because of old published
    date".

 3. We make extrainfo_insert() return a was_router_added_t, and we
    make its caller use it correctly.  This is probably redundant with
    the extrainfo_is_bogus flag.
Note that parse-list functions may add duplicate 'invalid' entries.
Add comments to can_dl_again usage
Merge remote-tracking branch 'public/bug11243_squashed'