Downgrade back to Tokio 0.2

[romac]

Close #699

• Referenced an issue explaining the need for the change
• Updated all relevant documentation in docs
• Updated all code comments where relevant
• Wrote tests
• Updated CHANGELOG.md

[cargo-dep-bot]

This PR made the following dependency changes:

Added Packages (Duplicate versions in '()'):
	h2 0.2.7 (0.1.26)
	mio-named-pipes 0.1.7
	pin-project 0.4.27 (1.0.2)
	pin-project-internal 0.4.27 (1.0.2)
	pin-project-lite 0.1.11 (0.2.0)
	tokio-util 0.3.1
	tracing-futures 0.2.4

Removed Packages (Remaining versions in '()'):
	bytes 0.6.0 (0.5.6, 0.4.12)
	mio 0.7.6 (0.6.22)
	ntapi 0.3.6

Updated Packages:
	async-tungstenite: 0.10.0 -> 0.9.3
	http-body: 0.4.0 -> 0.3.1
	hyper: 0.14.0-dev -> 0.13.9
	tokio: 0.3.4 -> 0.2.23
	tokio-macros: 0.3.1 -> 0.2.6

[codecov-io]

Codecov Report

Merging #700 (15055b1) into master (4c7e5d1) will decrease coverage by 0.0%.
The diff coverage is 30.0%.

@@           Coverage Diff            @@
##           master    #700     +/-   ##
========================================
- Coverage    41.7%   41.7%   -0.1%     
========================================
  Files         191     191             
  Lines       12588   12595      +7     
  Branches     3225    3223      -2     
========================================
+ Hits         5260    5262      +2     
- Misses       6986    6991      +5     
  Partials      342     342             

Impacted Files	Coverage Δ
light-client/src/utils/block_on.rs	0.0% <0.0%> (ø)
rpc-probe/src/error.rs	0.0% <0.0%> (ø)
rpc-probe/src/plan.rs	0.0% <0.0%> (ø)
rpc/src/client/transport/http.rs	0.0% <ø> (ø)
tendermint/tests/integration.rs	0.4% <0.0%> (ø)
rpc/src/client/subscription.rs	74.0% <100.0%> (ø)
rpc/src/client/transport/websocket.rs	73.0% <100.0%> (ø)
tendermint/src/config.rs	25.6% <0.0%> (-0.5%)	⬇️
light-client/src/macros.rs	11.1% <0.0%> (+<0.1%)	⬆️
rpc/src/response.rs	21.8% <0.0%> (+0.4%)	⬆️

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[xla]

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[melekes]

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