Implement Mutli-MerkleBucketDequeue (#2166)
Implement Mutli-MerkleBucketDequeue (#2166)

* Implement MultiMerkleBucketDequeue

Simplify merkle wrapping logic with spanner Keysets

* minimize diff

* Write the math in terms of suffixBuckets

* Implement limit

* move suffixBuckets

* Add TODO
gdbelvin/trillian,AlCutter/trillian,gdbelvin/trillian,gdbelvin/trillian,AlCutter/trillian,google/trillian,google/trillian