Improve logging in legacy megolm code (#3043)
* Use a PrefixedLogger throughout `megolm.ts`
Rather than manually adding `in ${this.roomId}` to each log line, use a
PrefixedLogger to achieve the same effect more consistently.
* Clean up logging in megolm.ts
Where we log a list of devices, we don't need the whole deviceinfo, just the
device id. All that noise makes it very hard to read the logs.
* Log users that we find in the room when encrypting
* Reduce log verbosity on decryption retries
