Change how MatrixEvent manages encrypted events
Make `MatrixEvent.event` contain the *encrypted* event, and keep the plaintext
payload in a separate `_clearEvent` property.
This achieves several aims:
* means that we have a record of the actual object which was received over
the wire, which can be shown by clients for 'view source'.
* makes sent and received encrypted MatrixEvents more consistent (previously
sent ones had `encryptedType` and `encryptedContent` properties, whereas
received ones threw away the ciphertext).
* Avoids having to make two MatrixEvents in the receive path, and copying
fields from one to the other.
*Hopefully* most clients have followed the advice given in the jsdoc of not
relying on MatrixEvent.event to get at events. If they haven't, I guess they'll
break in the face of encrypted events.
(The pushprocessor didn't follow this advice, so needed some tweaks.)
