Rework how SimpleCache works.
Rework how SimpleCache works.

Change the implementation of SimpleCache such that it does not rely upon
timers.  The problem with times is that they retain a reference to the
object even if Vimium itself loses all references.  This approach seems
simpler, and perfectly adequate for our purposes.

cyrixhero/vimium,elegantwww/vimium,mrmr1993/vimium,elderbas/vimium,gdh1995/vimium,poacher2k/vimium,justalittlenoob/vimium,elegantwww/vimium,willsALMANJ/vimium,borgified/vimium,zhangmin510/vimium,philc/vimium,dimatter/vimium,VPashkov/vimium,hhsue/vimium,poacher2k/vimium,zhangmin510/vimium,Robinson10240/vimium,decaffeinate-examples/vimium,PrestanceDesign/vimium,VPashkov/vimium,wildeyes/vimium,cyrixhero/vimium,justalittlenoob/vimium,Lw-Cui/vimium,mrmr1993/vimium,tuchangwei/vimium,decaffeinate-examples/vimium,PickRelated/vimium,PickRelated/vimium,PrestanceDesign/vimium,csmalin/vimium,smblott-github/vimium,bkudria/vimium,douglas-larocca/vimium,Aleroniponi/vimium,zhangmin510/vimium,dimatter/vimium,wildeyes/vimium,smblott-github/vimium,philc/vimium,wildeyes/vimium,PickRelated/vimium,tuchangwei/vimium,VPashkov/vimium,bkudria/vimium,Lw-Cui/vimium,cyrixhero/vimium,elderbas/vimium,Robinson10240/vimium,hhsue/vimium,Aleroniponi/vimium,bkudria/vimium,Robinson10240/vimium,douglas-larocca/vimium,willsALMANJ/vimium,smblott-github/vimium,justalittlenoob/vimium,elegantwww/vimium,gdh1995/vimium,elderbas/vimium,decaffeinate-examples/vimium,PrestanceDesign/vimium,philc/vimium,poacher2k/vimium,csmalin/vimium,dimatter/vimium,Aleroniponi/vimium,mrmr1993/vimium,Lw-Cui/vimium,borgified/vimium,willsALMANJ/vimium,tuchangwei/vimium,douglas-larocca/vimium,borgified/vimium,hhsue/vimium,gdh1995/vimium,csmalin/vimium