Currently GitLab serves requests to get raw blobs via Ruby-on-Rails code and
Unicorn. Because RoR/Unicorn is relatively heavyweight, in environment where
there are a lot of simultaneous requests to get raw blobs, this works very slow
and server is constantly overloaded.

On the other hand, to get raw blob content, we do not need anything from RoR
framework - we only need to have access to project git repository on filesystem,
and knowing whether access for getting data from there should be granted or
not. That means it is possible to handle '.../raw/....' request directly
in more lightweight and performant gitlab-workhorse.

As gitlab-workhorse is written in Go, and Go has good concurrency/parallelism
support and is generally much faster than Ruby, moving raw blob serving task to
it makes sense and should be a net win.

In this patch: we add infrastructure to process GET request for '/raw/...':

- extract project / ref and path from URL
- query auth backend for whether download access should be granted or not
- emit blob content via spawning external `git cat-file`

I've tried to mimic the output to be as close as the one emitted by RoR code,
with the idea that for users the change should be transparent.

As in this patch we do auth backend query for every request to get a blob, RoR
code is still loaded very much, so essentially there is no speedup yet:

  (on a 8-CPU i7-3770S with 16GB of RAM, 2001:67c:1254:e:8b::c776 is on localhost)

  # without patch: request eventually goes to unicorn  (9 unicorn workers)
  $ ./wrk -c40 -d10 -t1 --latency http://[2001:67c:1254:e:8b::c776]:7777/nexedi/slapos/raw/master/software/wendelin/software.cfg
  Running 10s test @ http://[2001:67c:1254:e:8b::c776]:7777/nexedi/slapos/raw/master/software/wendelin/software.cfg
    1 threads and 40 connections
    Thread Stats   Avg      Stdev     Max   +/- Stdev
      Latency   461.16ms   63.44ms 809.80ms   84.18%
      Req/Sec    84.84     17.02   131.00     80.00%
    Latency Distribution
       50%  460.21ms
       75%  492.83ms
       90%  524.67ms
       99%  636.49ms
    847 requests in 10.01s, 1.57MB read
  Requests/sec:     84.64
  Transfer/sec:    161.10KB

  # with this patch: request handled by gitlab-workhorse
  $ ./wrk -c40 -d10 -t1 --latency http://[2001:67c:1254:e:8b::c776]:7777/nexedi/slapos/raw/master/software/wendelin/software.cfg
  Running 10s test @ http://[2001:67c:1254:e:8b::c776]:7777/nexedi/slapos/raw/master/software/wendelin/software.cfg
    1 threads and 40 connections
    Thread Stats   Avg      Stdev     Max   +/- Stdev
      Latency   458.42ms   66.26ms 766.12ms   84.76%
      Req/Sec    85.38     16.59   120.00     82.00%
    Latency Distribution
       50%  459.26ms
       75%  490.09ms
       90%  523.95ms
       99%  611.33ms
    853 requests in 10.01s, 1.51MB read
  Requests/sec:     85.18
  Transfer/sec:    154.90KB

In the next patch we'll cache requests to auth backend and that will improve
performance dramatically.

NOTE 20160228: there is internal/git/blob.go trying to get raw data via
    gitlab-workhorse, but still asking Unicorn about blob->sha1 mapping
    etc. That work started in

        86aaa133 (Prototype blobs via workhorse, @jacobvosmaer)

    and was inspired by this patch. It goes out of line compared to what
    we can do if we serve all blob data just by gitlab-workhorse (see
    next patch), so we just avoid git/blob.go and put our stuff into
    git/xblob.go and tweak routes, essentially deactivating git/blob.go
    code.