Allows a datasource to return different chatItem instances for same message id

[diegosanchezr]

Previously, due to the mapping between chatItem instance and presenter, a new presenter would be created. This fixes:

1. That new presenter will have invalid itemVisibility property (would cause a crash when long pressing a text cell)
2. If a new chatItem instance is returned by the dataSource for a previous existing message, but the previous chatItem is leaked, then the presenter would leak too (and it might try to update its cell, as in downloading an image)

This introduces ChatItemCompanion, a structure which attaches a ChatItem with its presenter and decoration attributes. This solves 1) and 2)

Still, a new presenter will be created if a different instance is returned by the datasource for a same message id. This will cause its internal state to be lost, so it's recommended not to keep any state in the presenter, or use an external storage to be able to recover it. We might revisit this in the future by reusing same presenters for same uids or adding API to copy state from one presenter to a new one.

[codecov-io]

Current coverage is 61.85%

Merging #36 into dev will increase coverage by +0.56% as of f15b1ac

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  Stmts        2855    2894    +39
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+ Hit          1750    1790    +40
  Partial         0       0       
+ Missed       1105    1104     -1

Review entire Coverage Diff as of f15b1ac

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