* offchain: Simplify bits of http code

* offchain: Sort dev-dependencies

* deps: Bump parity-multiaddr to 0.7.3

Fixes build failure when using:
rustc 1.43.0-nightly (96bb8b31c 2020-03-05).

* offchain: Raise FD limit for HTTP tests

* offchain: Reword the comment on increasing the test fd limit