netstat to collect tcp connections per port

## Feature Request

### Proposal:
I'd like to see implemented the `netstat` plugin to collect the connection states per port that the system/a service listens on, just like the [TCPConns](https://collectd.org/wiki/index.php/Plugin:TCPConns) plugin of collectd

### Current behavior:
The netstat plugin only gathers the amount of state types.

### Desired behavior:
The netstat plugin gathers amount of state types per port.

### Use case:
Currently we're using the collectd plugin to show us how much connections each of our dovecot backend nodes is handling. We'd like to replace this with Telegraf.

Start by reading Telegraf's netstat plugin and the issue's reference to collectd's TCPConns plugin. Trace how the plugin currently counts connection state types, then determine how per-port state counts should be represented; done means the netstat plugin reports connection states for each listening port and covers the requested behavior with tests.

Written by the indexing model from the issue text.