JVMCBC-83: Use CopyOnWriteArraySet for less cpu usage.

Motivation
----------
When profiling the application on common workloads, it was discovered
that there were hot methods on the CHM iterator, traced back to the
RequestHandler's set of nodes.

Modifications
-------------
Using an optimized data structure for this workload (iterations vastly
outnumber the changes), using a CopyOnWriteArraySet instead of a backing
CHM, this CPU overhead is removed. The COWAS is also thread safe by
default, but it is using a simpler data structure undernath.

Result
------
Less CPU time spent when iterating the same data over and over.

Change-Id: I32af6b8862ddc5a9bb394e30f599e2f891cf1dd4
Reviewed-on: http://review.couchbase.org/44245
Tested-by: Michael Nitschinger <michael.nitschinger@couchbase.com>
Reviewed-by: Simon Baslé <simon@couchbase.com>