Enable eslint rules: @typescript-eslint/no-floating-promises

[hughns]

This builds on https://github.com/matrix-org/matrix-js-sdk/pull/2290 and is part of https://github.com/matrix-org/matrix-js-sdk/issues/2118

Rather than attempting to correctly fix the ambiguities that have been identified I have added TODO items to the codebase.

My suggestion is to merge this PR and then raise smaller PRs to address some of the suspect areas that might be causing race conditions.

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This change is marked as an internal change (Task), so will not be included in the changelog.

[codecov-commenter]

Codecov Report

Merging #2291 (af91c9a) into develop (5937e6a) will decrease coverage by 0.02%. The diff coverage is 38.28%.

@@             Coverage Diff             @@
##           develop    #2291      +/-   ##
===========================================
- Coverage    59.56%   59.54%   -0.03%     
===========================================
  Files           91       91              
  Lines        16416    16425       +9     
  Branches      3784     3784              
===========================================
+ Hits          9778     9780       +2     
- Misses        6638     6645       +7     

Impacted Files	Coverage Δ
src/http-api.ts	54.42% <0.00%> (ø)
src/interactive-auth.ts	67.50% <0.00%> (ø)
src/store/indexeddb.ts	1.11% <0.00%> (ø)
src/webrtc/callEventHandler.ts	7.69% <3.12%> (-0.11%)	:arrow_down:
src/models/relations.ts	50.33% <7.69%> (-0.34%)	:arrow_down:
src/webrtc/call.ts	36.07% <11.36%> (-0.05%)	:arrow_down:
src/crypto/dehydration.ts	11.71% <18.18%> (-0.11%)	:arrow_down:
src/client.ts	38.03% <25.00%> (ø)
src/models/room.ts	57.34% <40.00%> (ø)
src/crypto/verification/Base.ts	73.15% <50.00%> (ø)
... and 14 more

[hughns]

With regards to all the sites where errors are not handled. If it is the accepted design pattern is that a consumer of the API should register a global unhandledrejection handler then this should probably be stated in the documentation.

[hughns]

Cleaning up this stale PR. I would still advocate for the application of the @typescript-eslint/no-floating-promises rule on this project as it forces the explicit handling of promises.

[t3chguy]

@hughns I suggest bringing it up in a Web Weekly meeting where it can be voted upon