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:89::fa34 is on localhost)

  # without patch: request eventually goes to unicorn  (9 unicorn workers)
  $ ./wrk -c40 -d10 -t1 --latency http://[2001:67c:1254:e:89::fa34]:7777/root/slapos/raw/master/software/wendelin/software.cfg
  Running 10s test @ http://[2001:67c:1254:e:89::fa34]:7777/root/slapos/raw/master/software/wendelin/software.cfg
    1 threads and 40 connections
    Thread Stats   Avg      Stdev     Max   +/- Stdev
      Latency   609.34ms  156.92ms   1.18s    79.60%
      Req/Sec    64.22     19.90   120.00     67.00%
    Latency Distribution
       50%  596.50ms
       75%  678.23ms
       90%  805.72ms
       99%    1.04s
    642 requests in 10.01s, 1.24MB read
  Requests/sec:     64.16
  Transfer/sec:    127.00KB

  # with this patch: request handled by gitlab-workhorse
  $ ./wrk -c40 -d10 -t1 --latency http://[2001:67c:1254:e:89::fa34]:7777/root/slapos/raw/master/software/wendelin/software.cfg
  Running 10s test @ http://[2001:67c:1254:e:89::fa34]:7777/root/slapos/raw/master/software/wendelin/software.cfg
    1 threads and 40 connections
    Thread Stats   Avg      Stdev     Max   +/- Stdev
      Latency   622.99ms  200.08ms   1.40s    77.03%
      Req/Sec    62.65     22.37   120.00     55.00%
    Latency Distribution
       50%  589.51ms
       75%  726.88ms
       90%  896.09ms
       99%    1.18s
    626 requests in 10.01s, 1.11MB read
  Requests/sec:     62.55
  Transfer/sec:    113.73KB

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