Add HTTP request debug logging

[wfarner]

This adds debug-level logging for plugin clients and servers.

Here's an excerpt from the group Plugin to show how this logging presents, showing the full call chain to create an instance:

INFO[0000] Listening at: /Users/billfarner/.infrakit/plugins/group
DEBU[0015] Received request POST /rpc HTTP/1.1
Host: d
Accept-Encoding: gzip
Content-Length: 409
Content-Type: application/json
User-Agent: Go-http-client/1.1

{"method":"Group.CommitGroup","params":[{"Spec":{"ID":"cattle","Properties":{"Allocation":{"Size":5},"Instance":{"Plugin":"instance-file","Properties":{"Note":"Instance properties version 1.0"}},"Flavor":{"Plugin":"flavor-vanilla","Properties":{"Init":["docker pull nginx:alpine","docker run -d -p 80:80 nginx-alpine"],"Tags":{"tier":"web","project":"infrakit"}}}}},"Pretend":false}],"id":5577006791947779410}
...
DEBU[0015] Sending request POST / HTTP/1.1
Content-Type: application/json

{"method":"Flavor.Validate","params":[{"Properties":{"Init":["docker pull nginx:alpine","docker run -d -p 80:80 nginx-alpine"],"Tags":{"tier":"web","project":"infrakit"}},"Allocation":{"Size":5,"LogicalIDs":null}}],"id":2403489562072437156}
DEBU[0015] Received response HTTP/1.1 200 OK
Content-Length: 60
Content-Type: text/plain; charset=utf-8
Date: Tue, 15 Nov 2016 23:03:26 GMT

{"result":{"OK":true},"error":null,"id":2403489562072437156}
...
DEBU[0015] Sending request POST / HTTP/1.1
Content-Type: application/json

{"method":"Instance.Validate","params":[{"Properties":{"Note":"Instance properties version 1.0"}}],"id":6372761812113081704}
DEBU[0015] Received response HTTP/1.1 200 OK
Content-Length: 60
Content-Type: text/plain; charset=utf-8
Date: Tue, 15 Nov 2016 23:03:26 GMT

{"result":{"OK":true},"error":null,"id":6372761812113081704}
INFO[0015] Committing group cattle (pretend=false)
DEBU[0015] Sending response HTTP/1.1 200 OK
Connection: close
X-Content-Type-Options: nosniff

{"result":{"Details":"Managing 5 instances"},"error":null,"id":5577006791947779410}

Closes #269

Signed-off-by: Bill Farner bill@docker.com

[codecov-io]

Current coverage is 70.87% (diff: 75.00%)

Merging #300 into master will increase coverage by 0.03%

@@             master       #300   diff @@
==========================================
  Files            25         25          
  Lines          1228       1243    +15   
  Methods           0          0          
  Messages          0          0          
  Branches          0          0          
==========================================
+ Hits            870        881    +11   
- Misses          285        287     +2   
- Partials         73         75     +2   

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[GordonTheTurtle]

Please sign your commits following these rules:
https://github.com/docker/docker/blob/master/CONTRIBUTING.md#sign-your-work
The easiest way to do this is to amend the last commit:

$ git clone -b "rpc-log" git@github.com:wfarner/infrakit.git somewhere
$ cd somewhere
$ git rebase -i HEAD~842353570112
editor opens
change each 'pick' to 'edit'
save the file and quit
$ git commit --amend -s --no-edit
$ git rebase --continue # and repeat the amend for each commit
$ git push -f

Amending updates the existing PR. You DO NOT need to open a new one.

[chungers]

LGTM