Don't request multiple graphs at the same time.
Don't request multiple graphs at the same time.

Add a counter to keep track of the number of the number of pending
requests, and only allow one at a time to go to the server.  When
multiple requests are pending, only the last one will be executed,
others will be discarded.

Change-Id: Iac17572408869148bc737bee30c87e96993b520e

sidhhu/opentsdb,OpenTSDB/opentsdb,andyflury/opentsdb,sidhhu/opentsdb,alienth/opentsdb,ShefronYudy/opentsdb,OpenTSDB/opentsdb,ShefronYudy/opentsdb,eswdd/opentsdb,MadDogTechnology/opentsdb,MadDogTechnology/opentsdb,andyflury/opentsdb,johann8384/opentsdb,OpenTSDB/opentsdb,kidaa/opentsdb,kidaa/opentsdb,johann8384/opentsdb,eswdd/opentsdb,alienth/opentsdb,MadDogTechnology/opentsdb