devtools/slowproxy: add slowproxy tool to emulate slow network conditions

[mastercactapus]

Description:
Adds a new tool, slowproxy that can simulate network latency, jitter, and throughput constraints for testing.

Review
Add slowproxy to Procfile.local (create if necessary)

proxy: go run ./devtools/slowproxy -l localhost:3033 -c localhost:3030 -d 25ms -j 10ms

• run make start
• visit http://localhost:3033
• verify application works
• try other options to change behavior

$ go run ./devtools/slowproxy -h
Usage of /tmp/go-build2557664850/b001/exe/slowproxy:
  -c string
    	Server connect address. (default "localhost:5432")
  -d duration
    	Min latency (one-way).
  -i int
    	Max data rate (in bytes/sec) from server to client.
  -j duration
    	Jitter in (random +/- to latency).
  -l string
    	Listen address. (default "localhost:5435")
  -o int
    	Max data rate (in bytes/sec) from client to server.