* [p2p/pex] connect to more than 10 peers
also, remove DefaultMinNumOutboundPeers because a) I am not sure it's
needed b) it's super confusing
look closely
```
maxPeers := sw.config.MaxNumPeers - DefaultMinNumOutboundPeers
if maxPeers <= sw.peers.Size() {
sw.Logger.Info("Ignoring inbound connection: already have enough peers", "address", inConn.RemoteAddr().String(), "numPeers", sw.peers.Size(), "max", maxPeers)
```
we print maxPeers = config.MaxPeers - DefaultMinNumOutboundPeers. So we
may not have enough peers even though we say we have enough.
Refs #2130
* update spec
* replace MaxNumPeers with MaxNumInboundPeers/MaxNumOutboundPeers
Refs #2130
* update changelog
* make max rpc conns formula visible to users
* update spec
* docs: note max outbound peers excludes persistent
* limit the number of requests in flights for Prometheus server
Closes#1804
Default to 1 because usually there's just one collector.
* config: Up default for prom connections
Anton Kaliaev
bradyjoestar
Zach Ramsay
Jeremiah Andrews
Dev Ojha
Peng Zhong