Start loading next/prev message page earlier

[szuperaz]

Currently, the SDK loads the next/prev (depends on scroll direction) message page when the user scrolls to the very top/bottom of the channel (red line on the screenshot), but we can load it earlier, and then users won't have to wait (or wait less) for the messages to arrive (example line green on the screenshot).

[szuperaz]

Implementation is in progress, issue: on Safari the message list will load two pages instead of just one. This is related to a problem with scrollTop (that most likely not present in Google Chrome because that tries to preserve scroll position while scrolling anyway)