Prefer mmap()ed consensus files over cached_dir_t entries.

Cached_dir_t is a somewhat "legacy" kind of storage when used for
consensus documents, and it appears that there are cases when
changing our settings causes us to stop updating those entries.

This can cause trouble, as @arma found out in #40375, where he
changed his settings around, and consensus diff application got
messed up: consensus diffs were being _requested_ based on the
latest consensus, but were being (incorrectly) applied to a
consensus that was no longer the latest one.

This patch is a minimal fix for backporting purposes: it has Tor do
the same search when applying consensus diffs as we use to request
them.  This should be sufficient for correct behavior.

There's a similar case in GETINFO handling; I've fixed that too.

Fixes #40375; bugfix on 0.3.1.1-alpha.
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