Closes #415

Each channel now remembers an OwnedChanTarget.

Each circuit now remembers a vector of OwnedChanTarget to represent
the path that it was constructed for.

Part of #415.

This will help with #415

new api for isolation

See merge request tpo/core/arti!377

Absolish builders for CircMgrConfig and DirMgrConfig

See merge request tpo/core/arti!417

dirmgr: Always send if-modified-since on consensus documents.

Closes #403

See merge request tpo/core/arti!412

See rationale in the comment.

See commentary for the rationale.

Now we use NetParams.  That implies making its constructor public,
which I think it fine.

This is related to #413 but is far from completing that ticket.

This handwritten conversion function omitted a field.  There was
nothing to spot this mistake.

IMO this shows why these particular types ought not to use builders,
but instead, should cause API breaks when things change.

Adding this line here to explicitly fix the bug, although we are about
to abolish this function completely almost right away.

Fix typo

See merge request tpo/core/arti!416

it seems I added conditional compilation without noticing it??
and there was some errors when choosing a prefered runtime depending on
feature flags

this also removes JoinResult

the inverted logic was too easy to mess up

derive_builder: Switch to upstream 0.11

See merge request tpo/core/arti!414

This has the different syntax for builder field attributes than what I
originally proposed in my MR, and which therefore is in the pinned
branch.

My upstream MR for the field attributes feature was morged:
  https://github.com/colin-kiegel/rust-derive-builder/issues/239

The new version of derive_builder requires a newer darling.
Ooops.  Upstream MR here:
  https://github.com/colin-kiegel/rust-derive-builder/pull/242

For now, upgrade darling.  When there's a new upstream derive_builder,
we can use it and drop this.

Move RetryDelay into tor-basic-utils

See merge request tpo/core/arti!411

Replace a match with some as_ref and as

See merge request tpo/core/arti!413

Instead, check initial_delay in dirmgr directly.

The match was doing a deref coercion.

Found by current nightly's needless_match (whose suggestion doesn't
compile, https://github.com/rust-lang/rust-clippy/issues/8551 )

We never want a consensus document that's super-old, since we would
reject it immediately for being too old.

Also, never send an if-modified-since that's so old that we'd reject
the response.

Closes #403

It wasn't necessary.