Feature(session replay): add ability to visually reconstruct user sessions

[Jyyjy]

Copied from apache/flagon-useralejs#445

Problem

Session replay is a common feature in instrumentation services. As a developer, it's one of the nicest ways to make sense of user behavior and understand the context in which they encountered errors.

UserALE does not support the type of logs needed for this.

Solution

Use mutation observer to emit change events, works pretty well
Needs further research (look at [rrweb](https://www.rrweb.io/) as well as [Sentry](https://github.com/getsentry/sentry-javascript/tree/master/packages/replay-internal))
    Some problems with size of initial DOM payload