Spawn the syncer as an async task

[teor2345]

Motivation

Zebra's syncer currently runs in the initial async task on the main thread. This means that it is launched and terminated differently to other tasks.

It also shares the thread with more urgent tasks, such as interrupts and signals. But that should be handled by #1351.

Designs

• Make the PeerSet service sync and send (needs a Rust expert)
• Spawn the syncer in a separate task