Fix up instancepool example

This patch rewrites the instance pool example in C++ which has built in
STL types for managing lists and such, and allows the example to focus
more on the lcb-specific logic needed for instantiating a pool.

This rewrite also demonstrates how one might actually _use_ such a pool
by spinning up some "Worker" threads to perform operations.

Change-Id: If98c5263315a7c190726e411cf778b142c3a041c
Reviewed-on: http://review.couchbase.org/39933
Tested-by: Mark Nunberg <mnunberg@haskalah.org>
Reviewed-by: Brett Lawson <brett19@gmail.com>