BAC-318: add debugging logs to MW connection mgmt library

This change will add the following logs (wth PHP backtraces):

```
Jun  6 15:11:08 dev-macbre apache2[4227]: MWMemcached::_connect_sock-connected:my_wiki/:10.14.30.143:11000
LoadBalancer::reallyOpenConnection-connected: 10.14.30.145 [plpoznan]
LoadBalancer::reallyOpenConnection-connected: 10.14.30.142 [wikicities]
```

As it's going to be logged for every request, I'm still considering logging it only for small percentage of requests.

@federico-lox @owend @psychonaut @drozdo