fix: potential deadlock in pex module tests

[shotonoff]

Issue being fixed or feature implemented

The tests in pex module are flaky, i.e. TestReactorConnectFullNetwork

go test ./internal/p2p/pex -test.run TestReactorConnectFullNetwork -race -test.v -test.count 10
=== RUN   TestReactorConnectFullNetwork
    reactor_test.go:65: started ef59301921b33674348cc04ef4e63f8159bdb234
    reactor_test.go:65: started 395bc84197103724210f39587cb11a597c631696
    reactor_test.go:65: started 47c54b7311f120d81702f23b399761985df35fa4
    reactor_test.go:65: started 436ed2bf9bac168ca7f403c5f78c66e7fabc94d3
--- PASS: TestReactorConnectFullNetwork (2.02s)
=== RUN   TestReactorConnectFullNetwork
    reactor_test.go:65: started 21ac0dd98f38a817e15389961511803c164dfbb1
    reactor_test.go:65: started afaff974a583bcb46237ff53a1fc9d8f423b33d4
    reactor_test.go:65: started 37f264190bb20b4fb67c923f07f0818256262a87
    reactor_test.go:65: started d8e026d4a59747158fb3d03e1dbbc90f42943a49
POTENTIAL DEADLOCK:
Previous place where the lock was grabbed
goroutine 1663 lock 0xc0000ef778
../peermanager.go:851 p2p.(*PeerManager).Ready { m.mtx.Lock() } <<<<<
../peermanager.go:850 p2p.(*PeerManager).Ready { func (m *PeerManager) Ready(ctx context.Context, peerID types.NodeID, channels ChannelIDSet) { }
../router.go:722 p2p.(*Router).routePeer {  }

Have been trying to lock it again for more than 30s
goroutine 1267 lock 0xc0000ef778
../peermanager.go:581 p2p.(*PeerManager).TryDialNext { m.mtx.Lock() } <<<<<
../peermanager.go:580 p2p.(*PeerManager).TryDialNext { func (m *PeerManager) TryDialNext() NodeAddress { }
../peermanager.go:558 p2p.(*PeerManager).DialNext { for counter := uint32(0); ; counter++ { }
../router.go:557 p2p.(*Router).dialPeers { for { }

What was done?

Added ability to remove a subscription from the list through PeerManager.Unsubscribe

How Has This Been Tested?

Unit / E2E tests

Breaking Changes

N/A

Checklist:

• I have performed a self-review of my own code
• I have commented my code, particularly in hard-to-understand areas
• I have added or updated relevant unit/integration/functional/e2e tests
• I have made corresponding changes to the documentation

For repository code-owners and collaborators only

• I have assigned this pull request to a milestone