Add message id for to-device events

[Anderas]

Add automatically generated org.matrix.msgid to each to-device message (i.e. per user / device) as described in element-hq/element-meta#558

For context see Android and web implementation.

When sending to-device messages (e.g. room keys) the logs will look like this:

[MXMegolmEncryption] shareKey: Actually share with 2 users and 2 devices
[MXToDevicePayload] Created to-device payload with txnId m2054895014.1, message ids: [@charlie:x.y.z/IYYFXFKAIG 5A07D00E-CB64-4133-A1BB-F9CE1AFD38AA, @bob:x.y.z/CSCLHKJCVN 11C0E404-B0CC-433C-86EA-C883727C4CA7]
[MXHTTPClient] #105 - PUT _matrix/client/r0/sendToDevice/m.room.encrypted/m2054895014.1
[MXHTTPClient] #105 - PUT _matrix/client/r0/sendToDevice/m.room.encrypted/m2054895014.1 completed in 24ms
[MXMegolmEncryption] shareKey: request succeeded

When recieveing to-device messages the logs will look like this:

[MXSession] Received sync response in 50ms
[MXSession] handleSyncResponse: Received 0 joined rooms, 0 invited rooms, 0 left rooms, 1 toDevice events.
[MXSession] handleToDeviceEvents: Processing new to-device event msgid: 47B65E01-AEA2-4761-9E9F-B994A6E7B6D1
[MXSession] handleToDeviceEvents: Received new to-device event `m.room_key` from `@bob:x.y.z` msgid: 47B65E01-AEA2-4761-9E9F-B994A6E7B6D1
[MXMegolmDecryption] onRoomKeyEvent: Adding key for megolm session <id> from m.room_key event

Note that these logs will only work with legacy Crypto, whereas the work-in-progress Crypto V2 needs to make these changes in rust-sdk.

[BillCarsonFr]

LGTM