Fixed a memory leak in SchedulerProcess.decline.
Fixed a memory leak in SchedulerProcess.decline.

MesosScheduler.declineOffers has been changed ~6 months ago to send a
Decline message instead of calling acceptOffers with an empty list of
task infos. The changed version of declineOffer however did not remove
the offerId from the savedOffers map, causing a memory leak.

Review: https://reviews.apache.org/r/47804/

dforsyth/mesos,Gilbert88/mesos,dforsyth/mesos,verizonlabs/mesos,yuquanshan/customizedMesos,Aman-Jain-14/customizedMesos,zmalik/mesos,craimbert/mesos,neilconway/mesos,zmalik/mesos,craimbert/mesos,andschwa/mesos,craimbert/mesos,asamerh4/mesos,gsantovena/mesos,abudnik/mesos,yuquanshan/customizedMesos,zmalik/mesos,jpeach/mesos,gsantovena/mesos,abudnik/mesos,kaysoky/mesos,reneploetz/mesos,neilconway/mesos,reneploetz/mesos,craimbert/mesos,Gilbert88/mesos,dforsyth/mesos,neilconway/mesos,neilconway/mesos,chhsia0/mesos,jpeach/mesos,Aman-Jain-14/customizedMesos,andschwa/mesos,reneploetz/mesos,kaysoky/mesos,andschwa/mesos,Aman-Jain-14/customizedMesos,reneploetz/mesos,Aman-Jain-14/customizedMesos,asamerh4/mesos,chhsia0/mesos,chhsia0/mesos,gsantovena/mesos,yuquanshan/customizedMesos,reneploetz/mesos,andschwa/mesos,abudnik/mesos,Gilbert88/mesos,andschwa/mesos,verizonlabs/mesos,jpeach/mesos,jpeach/mesos,dforsyth/mesos,yuquanshan/customizedMesos,asamerh4/mesos,Gilbert88/mesos,zmalik/mesos,chhsia0/mesos,verizonlabs/mesos,Gilbert88/mesos,asamerh4/mesos,dforsyth/mesos,verizonlabs/mesos,craimbert/mesos,kaysoky/mesos,yuquanshan/customizedMesos,yuquanshan/customizedMesos,reneploetz/mesos,neilconway/mesos,zmalik/mesos,abudnik/mesos,gsantovena/mesos,asamerh4/mesos,Aman-Jain-14/customizedMesos,Gilbert88/mesos,verizonlabs/mesos,craimbert/mesos,gsantovena/mesos,yuquanshan/customizedMesos,craimbert/mesos,gsantovena/mesos,dforsyth/mesos,abudnik/mesos,abudnik/mesos,zmalik/mesos,Aman-Jain-14/customizedMesos,abudnik/mesos,andschwa/mesos,reneploetz/mesos,Aman-Jain-14/customizedMesos,verizonlabs/mesos,kaysoky/mesos,verizonlabs/mesos,Gilbert88/mesos,chhsia0/mesos,dforsyth/mesos,asamerh4/mesos,kaysoky/mesos,asamerh4/mesos,zmalik/mesos,kaysoky/mesos,jpeach/mesos,chhsia0/mesos,gsantovena/mesos,chhsia0/mesos,jpeach/mesos,jpeach/mesos,kaysoky/mesos,neilconway/mesos,andschwa/mesos,neilconway/mesos