Add workers middleware and reengineer diesel middleware to make use of it

[smangelsdorf]

The best (read: laziest) way to describe this PR is by pointing at the documentation for the "workers" and "diesel" middleware as written for this PR (mostly the rustdoc).

Ref #210

[smangelsdorf]

Ah, needs a rebase to work with Gotham master

[codecov]

Codecov Report

Merging #227 into master will increase coverage by 3.21%.
The diff coverage is 78.12%.

@@            Coverage Diff             @@
##           master     #227      +/-   ##
==========================================
+ Coverage    79.1%   82.31%   +3.21%     
==========================================
  Files          83       36      -47     
  Lines        3963     1674    -2289     
==========================================
- Hits         3135     1378    -1757     
+ Misses        828      296     -532

Impacted Files	Coverage Δ
...dleware/under_development/diesel/src/state_data.rs	100% <100%> (+100%)	⬆️
gotham/src/middleware/workers/pool.rs	76.66% <76.66%> (ø)
gotham/src/router/response/extender.rs	13.63% <0%> (-11.37%)	⬇️
gotham/src/service/timing.rs	56.52% <0%> (-5.98%)	⬇️
gotham/src/state/request_id.rs	97.56% <0%> (-2.44%)	⬇️
gotham/src/router/tree/node.rs	90.28% <0%> (-1.26%)	⬇️
gotham/src/test/request.rs	92.85% <0%> (-0.9%)	⬇️
gotham/src/router/route/matcher/accept.rs	0% <0%> (ø)	⬆️
misc/borrow_bag/src/append.rs	100% <0%> (ø)	⬆️
gotham_derive/src/extenders.rs	0% <0%> (ø)	⬆️
... and 68 more

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update d31462a...872906c. Read the comment docs.

[whitfin]

@smangelsdorf by any chance, do you want to finish this up? I think the workers middleware might be a good thing to ship alongside gotham itself, rather than being separated - we could maybe make use of it internally that way (and other people can work with it more easily).

[smangelsdorf]

Sure, will do. I kept it separate while it was an experiment but I'm happy to move it into the main Gotham codebase.

[smangelsdorf]

Done. Something's amiss with the clippy run, but otherwise all green.

[im-0]

@smangelsdorf, this happens because of my patches to run Clippy from Travis. #257 should fix this.

[smangelsdorf]

@im-0: Thanks, that sorted it 👍

[smangelsdorf]

I still plan to sort out the reduced coverage and the new merge conflict, just haven't got to it yet.

[smangelsdorf]

The changes here aren't compatible anymore with the master branch. The amount of work required to fix it probably warrants being submitted as a new PR (including weighing up whether the approach I took is still the correct one given the ecosystem as it exists now). I'm going to close this PR but leave the branch intact in case somebody wants to pick it up and continue working on it.

Recent releases of Rust may also allow some better refactoring, especially impl Trait which I note I've previously left a TODO comment about.

[whitfin]

@smangelsdorf noted - I'll take a look and it and see if I can get your work into master, to avoid having you wasted your time

[smangelsdorf]

The time is a sunk cost at this point, so don't worry about wasting it 😉

Hopefully some percentage of this is still useful in the new world of Tokio/Futures/Hyper, but if it's easier to throw it out and redo it differently I won't take it to heart.

[colinbankier]

@whitfin - I'm interested in seeing if I can resurrect this. I don't want to double up however, if you've already got far with it.