Update libp2p requirement from 0.18 to 0.19

[dependabot-preview[bot]]

Updates the requirements on libp2p to permit the latest version.

Changelog

Sourced from libp2p's changelog.

Version 0.19.0 (2020-05-18)

• libp2p-core, libp2p-swarm: Added support for multiple dialing attempts per peer, with a configurable limit. PR 1506

• libp2p-core: PeerIds that use the identity hashing will now be properly displayed using the string representation of an identity multihash, rather than the canonical SHA 256 representation. PR 1576

• libp2p-core: Updated to multihash 0.11.0. PR 1566

• libp2p-core: Make the number of events buffered to/from tasks configurable. PR 1574

• libp2p-dns, parity-multiaddr: Added support for the /dns multiaddr protocol. Additionally, the multiaddr::from_url function will now use /dns instead of /dns4. PR 1575

• libp2p-noise: Added the X25519Spec protocol suite which uses libp2p-noise-spec compliant signatures on static keys as well as the /noise protocol upgrade, hence providing a libp2p-noise-spec compliant XX handshake. IK and IX are still supported with X25519Spec though not guaranteed to be interoperable with other libp2p implementations as these handshake patterns are not currently included in the libp2p-noise-spec. The X25519Spec implementation will eventually replace the current X25519 implementation, with the former being removed. To upgrade without interruptions, you may temporarily include NoiseConfigs for both implementations as alternatives in your transport upgrade pipeline.

• libp2p-kad: Consider fixed (K_VALUE) amount of peers at closest query initialization. Unless KademliaConfig::set_replication_factor is used change has no effect. PR 1536

• libp2p-kad: Provide more insight into, and control of, the execution of queries. All query results are now wrapped in KademliaEvent::QueryResult. As a side-effect of these changes and for as long as the record storage API is not asynchronous, local storage errors on put_record are reported synchronously in a Result, instead of being reported asynchronously by an event. PR 1567

• libp2p-tcp, libp2p: Made the libp2p-tcp/async-std feature flag disabled by default, and split the libp2p/tcp feature in two: tcp-async-std and tcp-tokio. tcp-async-std is still enabled by default.

... (truncated)

Commits

• 34faf94 Publish 0.19.0 (#1579)
• 162a0be Display the identity hash of PeerIds when relevant (#1576)
• 2fbd1a5 Keep consistency with Upgrade Inbound by using PING_SIZE (#1578)
• bbc6735 Extend feature-flags to allow choosing runtime for libp2p-tcp (#1471)
• 82156de Support /dns protocol in multiaddr (#1575)
• 3a96ebf More insight into Kademlia queries. (#1567)
• c271f6f Make the number of events buffered to/from tasks configurable (#1574)
• 0443fea Update to latest multihash. (#1566)
• a83654c Add a method to create a pre shared key when you already have the raw bytes (...
• 0672112 protocols/kad: Remove unnecessary PeerId conversion for fixed iter (#1573)
• Additional commits viewable in compare view

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[dependabot-preview[bot]]

A newer version of libp2p exists, but since this PR has been edited by someone other than Dependabot I haven't updated it. You'll get a PR for the updated version as normal once this PR is merged.

[dependabot-preview[bot]]

OK, I won't notify you again about this release, but will get in touch when a new version is available. If you'd rather skip all updates until the next major or minor version, let me know by commenting @dependabot ignore this major version or @dependabot ignore this minor version.

If you change your mind, just re-open this PR and I'll resolve any conflicts on it.