Rethink MXKImageView: Improve performance and make it more specific

[SBiOSoftWhare]

MXKImageView is used for many cases, from displaying image in full screen to display read receipts avatars.
For example UIScrollView allocation in �initLayout method� is time consuming and not needed for most cases like avatar display.
I/O operations and network calls are also made on main thread.
Those issues slow down UI.

We should at least:

• Perform I/O operations and network calls in background
• Use a separate viewer for displaying images in full screen