server/: Make bulk crypto transforms responsible for algorithm selection.

server/: Make bulk crypto transforms responsible for algorithm selection.

Move all of the logic around processing symmetric algorithm selections
into the `bulkcrypto' transforms.  There are now three associated object
types:

  * an algorithm selection, which ends up attached to a peer key;

  * an encryption context, which actually performs the bulk transform on
    packets; and

  * a challenge context, which can issue and verify challenge tags.

The important improvement here is that now we can add new bulk crypto
transforms which are parametrized in different ways.