[hack] distinguish absent and deleted
[hack] distinguish absent and deleted

Summary:
Change `hh_mem_inner` so it distinguishes absent and removed keys.

internally,

1  -- present
-1 -- absent
-2 -- removed

Reviewed By: alexchow, dabek

Differential Revision: D7784882

fbshipit-source-id: ae663e8fc04e97abfcb1028b9ca58565c541685c

jamesgpearce/flow,JonathanUsername/flow,nmote/flow,gabelevi/flow,popham/flow,popham/flow,jamesgpearce/flow,gabelevi/flow,JonathanUsername/flow,gabelevi/flow,AgentME/flow,mroch/flow,popham/flow,mroch/flow,gabelevi/flow,JonathanUsername/flow,popham/flow,AgentME/flow,mroch/flow,AgentME/flow,nmote/flow,AgentME/flow,samwgoldman/flow,jamesgpearce/flow,facebook/flow,samwgoldman/flow,JonathanUsername/flow,popham/flow,facebook/flow,nmote/flow,facebook/flow,claudiopro/flow,samwgoldman/flow,jamesgpearce/flow,gabelevi/flow,mroch/flow,facebook/flow,mroch/flow,mroch/flow,jamesgpearce/flow,claudiopro/flow,popham/flow,popham/flow,claudiopro/flow,gabelevi/flow,mroch/flow,nmote/flow,nmote/flow,JonathanUsername/flow,samwgoldman/flow,AgentME/flow,JonathanUsername/flow,gabelevi/flow,samwgoldman/flow,nmote/flow,jamesgpearce/flow,facebook/flow,facebook/flow,claudiopro/flow,samwgoldman/flow,claudiopro/flow,samwgoldman/flow,jamesgpearce/flow,facebook/flow,JonathanUsername/flow,nmote/flow