Remove redundant watch values once our Tokio dependency is updated

[jvff]

Motivation

When Zebra uses tokio::sync::watch::Sender, we sometimes need an extra value field, because Tokio version 0.3.6, doesn't have a watch::Sender::borrow method.

That method is available on Tokio 1.0.0, so once we update to the newer version we can remove that field and replace its usage with *self.sender.borrow().

Impacted types:

• ChainTipSender
  • Reject connections from outdated peers #2519
  • Refactor BestTipHeight into a generic ChainTip sender and receiver  #2676
• RecentSyncLengths
  • Provide recent syncer response lengths as a watch channel #2602

Related Work

#2200 tracks updating the Tokio dependency.

[teor2345]

We can fix this when we change the related code.