Make a number of the crypto APIs asynchronous
Make the following return Promises:
* `MatrixClient.getStoredDevicesForUser`
* `MatrixClient.getStoredDevice`
* `MatrixClient.setDeviceVerified`
* `MatrixClient.setDeviceBlocked`
* `MatrixClient.setDeviceKnown`
* `MatrixClient.getEventSenderDeviceInfo`
* `MatrixClient.isEventSenderVerified`
* `MatrixClient.importRoomKeys`
Remove `listDeviceKeys` altogether: it's been deprecated for ages, and since
applications are going to have to be changed anyway, they might as well use its
replacement (`getStoredDevices`).