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

Author: richvdh

spec/unit/crypto/algorithms/megolm.spec.ts

@@ -387,8 +387,10 @@ describe("MegolmDecryption", function () {
                 mockCrypto.baseApis = mockBaseApis;
 
                 mockRoom = {
+                    roomId: ROOM_ID,
                     getEncryptionTargetMembers: jest.fn().mockReturnValue([{ userId: "@alice:home.server" }]),
                     getBlacklistUnverifiedDevices: jest.fn().mockReturnValue(false),
+                    shouldEncryptForInvitedMembers: jest.fn().mockReturnValue(false),
                 } as unknown as Room;
             });