server/: New `implicit-IV' transform (`iiv').

The new transform has two advantages over the existing `v0' transform:

  * it doesn't need to transmit an explicit IV, so it adds less overhead
    to messages being sent; and

  * it's entirely deterministic, getting variation from the input
    sequence number rather than randomness, which (a) improves
    performance a bit by not exercising the cryptographic random number
    generator, and (b) eliminates a kleptographic channel.

This change triggers a bug in mLib 2.2.1 and earlier: `dstr_putf'
doesn't handle `%.*s' correctly.