* Rename some methods and constants for clarity
Using the following commands:
```
fastmod '\bis_ready_for_attempt\b' is_ready_for_connection_attempt
# One instance required a tweak, because of the ASCII diagram.
fastmod '\bwas_recently_live\b' has_connection_recently_responded
fastmod '\bwas_recently_attempted\b' was_connection_recently_attempted
fastmod '\bwas_recently_failed\b' has_connection_recently_failed
fastmod '\bLIVE_PEER_DURATION\b' MIN_PEER_RECONNECTION_DELAY
```
* Use `Instant::elapsed` for conciseness
Instead of `Instant::now().saturating_duration_since`. They're both
equivalent, and `elapsed` only panics if the `Instant` is somehow
synthetically generated.
* Allow `Duration32` to be created in other crates
Export the `Duration32` from the `zebra_chain::serialization` module.
* Add some new `Duration32` constructors
Create some helper `const` constructors to make it easy to create
constant durations. Add methods to create a `Duration32` from seconds,
minutes and hours.
* Avoid gossiping unreachable peers
When sanitizing the list of peers to gossip, remove those that we
haven't seen in more than three hours.
* Test if unreachable addresses aren't gossiped
Create a property test with random addreses inserted into an
`AddressBook`, and verify that the sanitized list of addresses does not
contain any addresses considered unreachable.
* Test if new alternate address isn't gossipable
Create a new alternate peer, because that type of `MetaAddr` does not
have `last_response` or `untrusted_last_seen` times. Verify that the
peer is not considered gossipable.
* Test if local listener is gossipable
The `MetaAddr` representing the local peer's listening address should
always be considered gossipable.
* Test if gossiped peer recently seen is gossipable
Create a `MetaAddr` representing a gossiped peer that was reported to be
seen recently. Check that the peer is considered gossipable.
* Test peer reportedly last seen in the future
Create a `MetaAddr` representing a peer gossiped and reported to have
been last seen in a time that's in the future. Check that the peer is
considered gossipable, to check that the fallback calculation is working
as intended.
* Test gossiped peer reportedly seen long ago
Create a `MetaAddr` representing a gossiped peer that was reported to
last have been seen a long time ago. Check that the peer is not
considered gossipable.
* Test if just responded peer is gossipable
Create a `MetaAddr` representing a peer that has just responded and
check that it is considered gossipable.
* Test if recently responded peer is gossipable
Create a `MetaAddr` representing a peer that last responded within the
duration a peer is considered reachable. Verify that the peer is
considered gossipable.
* Test peer that responded long ago isn't gossipable
Create a `MetaAddr` representing a peer that last responded outside the
duration a peer is considered reachable. Verify that the peer is not
considered gossipable.
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