Add tracking for stats similar to ChannelMetrics per peer

The node as of version 1.6 tracks metrics per communication channel (which are multiplexed over the regular connection pair) for the entire node. We should add a set of metrics that is similar on a per-client basis, to aid in debugging potentially problematic peers. Access to these metrics should be given only via the diagnostics port for now.
For the implementation, one can either try a lock-based or atomics based approach. In the former case, a new stats structure should be created that contains a simple set of statistics to gather behind an RwLock or similar. With atomics, the existing ChannelMetrics could probaby be reused.
Care should be given to the total increase in memory used and heap allocations, as this is per connection, of which we can potentially have 1000s of.