Monitor/Mutator: Cleanup handling of epoch 1

[liamsi]

Currently, mutations can be requested for epochs starting from epoch 1. Epoch 1 is created initially by the sequencer if no previous epoch exists and the tree is still empty (no mutations exist).
The monitor currently needs to request epoch 1 which fails when inclusions proofs are setup to point the previous epoch which is the expected behavior (epoch 0 does not exist).

With

keytransparency/core/mutation/mutation.go

Lines 109 to 115 in 9b99af5

	var epoch int64
	if in.Epoch > 1 {
	epoch = in.Epoch - 1
	} else {
	epoch = 1
	}
	proofs, err := s.inclusionProofs(ctx, indexes, epoch)

we introduced a temporary workaround/hack that makes the API work even if epoch 1 (mutation do not exist) is requested.

On the long-run we rather want:

• epoch 2 is the first epoch we want to allow the mutation service to allow as a valid request (mutations might exist)
• use/provide another API to request epoch 1

[gdbelvin]

This has been resolved in the following way:

• During domain initialization, the first map root is placed in the trillian log so that log.tree_size should always >= 1.
• Epoch == mapRoot.Revision. Which starts at revision 0.

#966 formalizes checking the latest revision during GetLatestEpoch