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romanb Kademlia: Somewhat complete the records implementation. (#1189)
* 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]: #146
[libp2p-1089]: #1089
[libp2p-go-323]: libp2p/go-libp2p-kad-dht#323
[libp2p-js-98]: libp2p/js-libp2p-kad-dht#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 .
Latest commit cde93f5 Jul 17, 2019
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