Fix potential deadlock in registry changes communication

[stanislav-zeman]

The change propagation (i.e. the channel communication) has to be somehow reworked. I currently "resolved" the issue in Consul registry by using buffered channel. Please note that if any service has the "delta" of instances (number of changes processed by writeChanges() method) greater than the channel buffer size it will deadlock. This is caused by the resolver which calls the Instances() method for each change which tries to read-lock the registry mutex. This is problematic in conjunction with the locking in the writeChanges() method which writes to this channel under write lock.

One potential fix could be communicating the changes in batches instead of sending message per service Registered/Unregistered state.