* Somewhat complete the implementation of Kademlia records.
This commit relates to [libp2p-146] and [libp2p-1089].
* All records expire (by default, configurable).
* Provider records are also stored in the RecordStore, and the RecordStore
API extended.
* Background jobs for periodic (re-)replication and (re-)publication
of records. Regular (value-)records are subject to re-replication and
re-publication as per standard Kademlia. Provider records are only
subject to re-publication.
* For standard Kademlia value lookups (quorum = 1), the record is cached
at the closest peer to the key that did not return the value, as per
standard Kademlia.
* Expiration times of regular (value-)records is computed exponentially
inversely proportional to the number of nodes between the local node
and the closest node known to the key (beyond the k closest), as per
standard Kademlia.
The protobuf messages are extended with two fields: `ttl` and `publisher`
in order to implement the different semantics of re-replication (by any
of the k closest peers to the key, not affecting expiry) and re-publication
(by the original publisher, resetting the expiry). This is not done yet in
other libp2p Kademlia implementations, see e.g. [libp2p-go-323]. The new protobuf fields
have been given somewhat unique identifiers to prevent future collision.
Similarly, periodic re-publication of provider records does not seem to
be done yet in other implementations, see e.g. [libp2p-js-98].
[libp2p-146]: https://github.com/libp2p/rust-libp2p/issues/146
[libp2p-1089]: https://github.com/libp2p/rust-libp2p/issues/1089
[libp2p-go-323]: https://github.com/libp2p/go-libp2p-kad-dht/issues/323
[libp2p-js-98]: https://github.com/libp2p/js-libp2p-kad-dht/issues/98
* Tweak kad-ipfs example.
* Add missing files.
* Ensure new delays are polled immediately.
To ensure task notification, since `NotReady` is returned right after.
* Fix ipfs-kad example and use wasm_timer.
* Small cleanup.
* Incorporate some feedback.
* Adjustments after rebase.
* Distinguish events further.
In order for a user to easily distinguish the result of e.g.
a `put_record` operation from the result of a later republication,
different event constructors are used. Furthermore, for now,
re-replication and "caching" of records (at the closest peer to
the key that did not return a value during a successful lookup)
do not yield events for now as they are less interesting.
* Speed up tests for CI.
* Small refinements and more documentation.
* Guard a node against overriding records for which it considers
itself to be the publisher.
* Document the jobs module more extensively.
* More inline docs around removal of "unreachable" addresses.
* Remove wildcard re-exports.
* Use NonZeroUsize for the constants.
* Re-add method lost on merge.
* Add missing 'pub'.
* Further increase the timeout in the ipfs-kad example.
* Readd log dependency to libp2p-kad.
* Simplify RecordStore API slightly.
* Some more commentary.
* Change Addresses::remove to return Result<(),()>.
Change the semantics of `Addresses::remove` so that the error case
is unambiguous, instead of the success case. Use the `Result` for
clearer semantics to that effect.
* Add some documentation to .
