Fixed warning in `process/timer.hpp`.
Fixed warning in `process/timer.hpp`.

We were unnecessarily casting from `long` to `uint64_t`.

Review: https://reviews.apache.org/r/52201/

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