Optimize gossip score calculation

[Nashatyrev]

Description

On every inbound message GossipRouter checks the peer score against graylistThreshold to decide whether a message should be processed or ignored.
The score calculation is not too cheap for extensive inbound message flow.

The following optimizations were made:

• Cache PeerHandler.getIP()
• Cache topic score unless any of input values changed or time changed more than timeInMeshQuantum. Ideally that should be made on something like Rx ObservableValues and Bindings to look nicer but that seems like an overkill for this change
• Maintain whitelist with expiration and message count for acceptRequestsFrom for peers with non-negative score to avoid score recalculation for every inbound message. The basic idea behind is that graylistThreshold should be far below 0 and if a peer has the score 0 or above we could unconditionally accept some reasonable amount of its messages for say 1 second.

Performance

Profiling graph of AbstractRouter.onInbound()

Before

After