Track the number of active inbound and outbound peer connections

[teor2345]

Motivation

To limit the number of connections in Zebra, we need to be able to track how many open connections there are.

Solution

• Track the number of open connections using a new ActiveConnectionCounter type
• Create a ConnectionTracker type that increases the count when it's created, and decreases the count when it's dropped
• Increase the count before opening new connections
• Reduce the connection count when Connections close or fail, by adding a ConnectionTracker to each Connection

Closes #2903.
Closes #2904.

This PR will get tested as part of tickets #1850 and #2902, when we actually enforce connection limits.

Review

@oxarbitrage and @dconnolly wanted to work on the security tickets that depend on this PR.
So they probably want to review it.

This PR is on the critical path for the Zebra beta, so it's urgent.

Reviewer Checklist

Security edge-cases:

• Zebra should check and increase the connection limit as early as possible, before it uses any resources for the connection
• Zebra should send the drop signal as late as possible, after it has dropped everything else that's used by the connection
• How can we make sure the inbound and outbound signals don't block each other?
  • They use completely separate channel instances via a wrapper type.
• How can we make sure we don't get the inbound and outbound signals mixed up?
  • Separate channels are created within the inbound and outbound tasks. They aren't shared.

Follow Up Work

• Security: Limit the number of outbound peer connections when dialing new peers #1850
  • Limit the initial crawler demand for outbound peers, based on seed peer connections #2902

[oxarbitrage]

I think this looks great.