Made sure process::Loop instances can only be created as shared_ptrs.
Made sure process::Loop instances can only be created as shared_ptrs.

process::Loop is a std::enable_shared_from_this. Using stack-allocated
instances of classes derived from std::enable_shared_from_this is
undefined behavior.

Prevent creation of stack-allocated process::Loop instances by hiding
the constructors and providing a factory method to be used instead.

Review: https://reviews.apache.org/r/54825/

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