encryptMessageFor now requires an array buffer instead of a protobuf.
By converting the message to an array buffer outside the Promise.all, we
avoid allocating a new copy for each device.
// FREEBIE
Previously we would convert a bytebuffer to a string, pass it to
libaxolotl where it would be parsed back into a bytebuffer.
Ideally we would just pass the bytebuffer, but it turns out that
libaxolotl's bytebyffer class is identical but separate from
libtextsecure's bytebuffer class. ¯\_(ツ)_/¯
So instead we pass the underlying array buffer, which is handled
more or less the same way as a bytebuffer, and most importantly,
does not involve any copying.
// FREEBIE
