Based rust nightly cargo fmt.

Now we are testing decompression of the input via calling
the process method 3 times. We get the final decompressed output
only after the third call.

Tests the decompression of the whole input in one go.

Adds dependency to zstd crate.

asynchronous_codec is a fork of futures_codec that is up-to-date on
its dependencies.

This migration allows us to upgrade to the current version of the
bytes crate.

Co-authored-by: David Goulet <dgoulet@torproject.org>

Co-authored-by: David Goulet <dgoulet@torproject.org>

These necessitated a little complexity for our circuit
creation/extension API; we might want to refactor that down the
road.

Closes arti#49.

Found by fuzzing: without doing this, there are ways to make the
SocksHandshake panic.  I don't think any of them are reachable from
the network assuming this crate is used correctly, but let's be safer.

Had to refactor a few methods for database access, and fix a bug
where we were storing valid_until times with the wrong information.