Firecracker release v0.20.0

[serban300]

Reason for This PR

Firecracker release v0.20.0

Description of Changes

### Added

- Added support for GICv2.

### Fixed

- Fixed a logical error in bounds checking performed on vsock virtio
  descriptors (CVE-2019-18960).
- Fixed #1283 - Can't start a VM in AARCH64 with vcpus number more than 16.
- The backtrace are printed on `panic`, no longer causing a seccomp fault.
- Fixed #1375 - Change logger options type from Value to Vec<LogOption> to
  prevent potential unwrap on None panics.
- Raise interrupt for TX queue used descriptors - Github issue #1436
- Fixed a bug that causes 100% cpu load when the net device rx is throttled
  by the ratelimiter - Github issue #1439
- Invalid fields in rate limiter related API requests are now failing with
  a proper error message.
- Fixed #1316 - fix the size of a virtio device, backed by a block device.
- Log failed api requests.

### Changed

- Decreased release binary size by 10%.

License Acceptance

By submitting this pull request, I confirm that my contribution is made under
the terms of the Apache 2.0 license.

PR Checklist

[Author TODO: Meet these criteria. Where there are two options, keep one.]
[Reviewer TODO: Verify that these criteria are met. Request changes if not]

• All commits in this PR are signed (git commit -s).
• Either this PR is linked to an issue, or, the reason for this PR is
  clearly provided.
• The description of changes is clear and encompassing.
• Either no docs need to be updated as part of this PR, or, the required
  doc changes are included in this PR. Docs in scope are all *.md files
  located either in the repository root, or in the docs/ directory.
• Either no code has been touched, or, code-level documentation for touched
  code is included in this PR.
• Either no API changes are included in this PR, or, the API changes are
  reflected in firecracker/swagger.yaml.
• Either the changes in this PR have no user impact, or, the changes in
  this PR have user impact and have been added to the CHANGELOG.md file.
• Either no new unsafe code has been added, or, the newly added unsafe
  code is unavoidable and properly documented.

[sandreim]

Please delay the release so I can merge a fix for #1436.

[andreeaflorescu]

Can we resume the work on this release? All relevant blockers seem to be merged now.

All blocker PRs merged.

[dianpopa]

I d like for us to use the same format across the entire changelog when associating a issue to a fixed entry.
Now we have

Fixes #12...
- GitHub issue #12...
(GitHub issue #12...)