[ISSUE-97] Adding max upstream connection configuration

[jlindenbaum]

As discussed in #97.

Adds a max_upstream_connections options that will fail creating additional upstream connections from being made. This is an attempt to preserve the upstream from becoming overwhelmed.

Questions / Things that are broken:

• I have not been able to write a good test for this. Is there a better mechanism to "trick" the active client to use a new upstream connection in the spec?
• I am unsure what the best closing frame would be to send to the client (server.cr::91 TODO)

[carlhoerberg]

I think this is less needed now that we do channel pooling, very few upstream connections should be needed now (most often only 1).