NCBC-513: Invalid timespan calculation in MemcachedClient.cs

Change-Id: Iccd14e601e73c311770029ed0be1eabf23c2aeb3
Reviewed-on: http://review.couchbase.org/38994
Tested-by: Jeffry Morris <jeffrymorris@gmail.com>
Reviewed-by: Brett Lawson <brett19@gmail.com>