Threads: added support to read receipts (MSC3771)

[gileluard]

needed for element-hq/element-ios#6663

[ismailgulek]

Had some inline comments, otherwise lgtm

[ismailgulek]

Why don't we use kMXEventTimelineMain instead of NSNull?

[gileluard]

We still must support RR without thread ID (unthreaded RR) sent by old clients. In this case we have to set the thread ID as nil so the RR will be stored in all thread and main timelines as per MSC.

[ismailgulek]

A tiny comment inline, otherwise LGTM!

[codecov]

Codecov Report

Base: 16.16% // Head: 16.07% // Decreases project coverage by -0.08% ⚠️

Coverage data is based on head (03d3104) compared to base (cf61531).
Patch coverage: 2.09% of modified lines in pull request are covered.

Additional details and impacted files

@@             Coverage Diff             @@
##           develop    #1617      +/-   ##
===========================================
- Coverage    16.16%   16.07%   -0.09%     
===========================================
  Files          580      581       +1     
  Lines        91432    91980     +548     
  Branches     38555    38807     +252     
===========================================
+ Hits         14777    14789      +12     
- Misses       76176    76711     +535     
- Partials       479      480       +1     

Impacted Files	Coverage Δ
MatrixSDK/Background/MXBackgroundStore.swift	0.00% <0.00%> (ø)
MatrixSDK/Contrib/Swift/MXRestClient.swift	1.55% <0.00%> (ø)
...xSDK/Data/EventTimeline/Room/MXRoomEventTimeline.m	5.24% <0.00%> (-0.03%)	⬇️
MatrixSDK/Data/MXRoom.m	1.68% <0.00%> (-0.03%)	⬇️
MatrixSDK/Data/MXRoomSummary.m	7.67% <0.00%> (ø)
MatrixSDK/Data/Store/MXFileStore/MXFileStore.m	17.99% <0.00%> (-0.03%)	⬇️
MatrixSDK/Data/Store/MXMemoryStore/MXReceiptData.m	0.00% <0.00%> (ø)
MatrixSDK/JSONModels/MXEvent.m	34.06% <0.00%> (-1.20%)	⬇️
MatrixSDK/MXRestClient.m	2.45% <0.00%> (-0.01%)	⬇️
MatrixSDK/Threads/MXThreadingService.swift	19.67% <0.00%> (-0.61%)	⬇️
... and 3 more

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