Use only a single callback per MultiAction.
Use only a single callback per MultiAction.

This divides by 2 the number of internal callbacks
we create to handle MultiAction responses.

Change-Id: I1dd503de291f11dd19da325be95185c5567c2bd0

StumbleUponArchive/asynchbase,angushe/asynchbase,manolama/asynchbase,hzy001/asynchbase,Connexity/asynchbase,angushe/asynchbase,tempbottle/asynchbase,manolama/asyncbigtable,mapr/asynchbase,hhshuai/asynchbase,SteamShon/asynchbase,StumbleUponArchive/asynchbase,yuzhu712/asynchbase,adityakishore/asynchbase,mway08/asynchbase,hzy001/asynchbase,tsuna/asynchbase,tsuna/asynchbase,Connexity/asynchbase,yuzhu712/asynchbase,OpenTSDB/asyncbigtable,mapr/asynchbase,SteamShon/asynchbase,nvoron23/asynchbase,OpenTSDB/asynchbase,hhshuai/asynchbase,OpenTSDB/asynchbase,mway08/asynchbase,nvoron23/asynchbase,manolama/asynchbase,pythian/asynchbase,tempbottle/asynchbase,adityakishore/asynchbase