Add command to get current sessions for server in backend

[mchugh19]

Currently, not all haproxy stats are accessible from haproxyctl. This code adds a new command to query the number of current sessions for a specified server from a specified backend.

This fits my needs, but perhaps consideration should be given to a more general stats reporter?

[neurogeek]

Hi @mchugh19, that looks good and I'll gladly merge it. Would you mind taking a look at my comments? Thanks for the contribution!