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

This patch can be tested by building dbcore using the latency_monkey branch of rexi [1], setting the appropriate config parameters and calling the appropriate fabric functions in a remsh [2].

[1] https://github.com/cloudant/rexi/tree/26984-latency-monkey

[2] https://gist.github.com/mikewallace1979/43db43c4506deff372e3