Make sc-offchain tests more resilient on macOS (#5191)

* 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

substrate/Cargo.lock

@@ -2642,7 +2642,7 @@ dependencies = [
  "libp2p-wasm-ext",
  "libp2p-websocket",
  "libp2p-yamux",
- "parity-multiaddr 0.7.2",
+ "parity-multiaddr 0.7.3",
  "parity-multihash 0.2.3",
  "parking_lot 0.10.0",
  "pin-project",
@@ -2666,7 +2666,7 @@ dependencies = [
  "libsecp256k1",
  "log 0.4.8",
  "multistream-select",
- "parity-multiaddr 0.7.2",
+ "parity-multiaddr 0.7.3",
  "parity-multihash 0.2.3",
  "parking_lot 0.10.0",
  "pin-project",
@@ -4585,9 +4585,9 @@ dependencies = [
 
 [[package]]
 name = "parity-multiaddr"
-version = "0.7.2"
+version = "0.7.3"
 source = "registry+https://github.com/rust-lang/crates.io-index"
-checksum = "26df883298bc3f4e92528b4c5cc9f806b791955b136da3e5e939ed9de0fd958b"
+checksum = "f77055f9e81921a8cc7bebeb6cded3d128931d51f1e3dd6251f0770a6d431477"
 dependencies = [
  "arrayref",
  "bs58 0.3.0",
@@ -6281,6 +6281,7 @@ version = "2.0.0-alpha.3"
 dependencies = [
  "bytes 0.5.4",
  "env_logger 0.7.1",
+ "fdlimit",
  "fnv",
  "futures 0.3.4",
  "futures-timer 3.0.2",