Consistently log fabric worker timeouts

Write a log line for each worker that did not return a response
when a fabric request times out. The format of the log line is:

    fabric_worker_timeout ENDPOINT,NODE,SHARD_NAME

This is intented to be easily consumable by downstream tools
(e.g., Splunk).

BugzID: 26984
Merge pull request #148 from cloudant/26984-log-fabric-timeouts

Consistently log fabric worker timeouts