Consolidate remote_proxy and recordshell

I think reviewing commits: https://github.com/ravinet/mahimahi/commit/c5bbdbc31f970653688d412a0483b77c69a0136a (change to HTTPDiskStore ctor) and https://github.com/ravinet/mahimahi/commit/fbaf8fe5f0567986cd303c14322927acd19b8aeb (making ProcessRecorder parametric) would be useful.

I wasn't sure how much logic to put into the ctor for ProcessRecorder vs the record_process method, and right now almost everything is in the record_process method, except for the creation of the HTTPDiskStore. This division of labor is somewhat arbitrary and I can refactor the code if needed to move everything into the ctor or everything into the record_process method as required.