Changed start scripts to use concurrent mark/sweep GC for proto.

The proto frontend was pausing for long periods towards the end of
experiments running at higher load. This commit changes the
start_proto_frontend.sh script to use concurrent mark and sweep GC
for the proto frontend (just like we do for sparrow), and also adds
GC logging, in an attempt to fix the problem.
Switched to using TThreadPoolServer, which performs marginally better.