remote: sideband support

[mcuadros]

This is the support of Remote for sideband capable servers.

When Remote. Fetch is called If the request contained a side-band-64k or side-band a sideband.Demux is used inside of Remote to read the packfile data.

If the human readable progress is needed a io.Reader/io.Writer should be provided to Repository.Progress field, this pointer will be pass to the remotes and be used when the packfile is read. If the Repository.Progress is not set the no-progress capability is set if supported.

Also a couple of bugs are solved at transport/client:

• Content-Length is correctly encoded using strconv.Itoa instead of just casting a integer to string.
• Request encoding is done now using the encoders provide by the `protocol/packp package

I don't know how I ended being impossible to push to the branch, so this is a fork in another org, so is a new PR of #156

remote: Resolving deltas: 100% (32/32), completed with 20 local objects.
remote: Unexpected system error after push was received.
remote: These changes may not be reflected on github.com!
remote: Your unique error code: a3244d2900ca54a5f82ef2504896b2ed

[codecov-io]

Current coverage is 76.91% (diff: 77.55%)

Merging #164 into master will decrease coverage by 0.79%

@@             master       #164   diff @@
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  Files            88         88          
  Lines          5475       5489    +14   
  Methods           0          0          
  Messages          0          0          
  Branches          0          0          
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- Hits           4255       4222    -33   
- Misses          761        815    +54   
+ Partials        459        452     -7   

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