[shutter] explore refactoring sentinel to fit needs of shutter and caplin in an agnostic way

[taratorio]

might be quite a bit of work for little gain - leaving here as a brain dump future follow up for now

can follow options pattern for sentinel and create options for logical bits that we can extract out of sentinel such as:

• stream handlers
• custom libp2p options such as whether to use go-libp2p-kad-dht with pubsub for better discovery
• option whether to persist peers in a persistent store instead of in-memory
• option whether to use identity from nodekey file
• etc.