SubtreeCache: Batch tile reads when writing nodes

[pav-kv]

The SubtreeCache uses two types of "read" callbacks: GetSubtreeFunc for
single-tile reads, and GetSubtreesFunc for batched reads. Besides being
complex, this also has performance issues: the single-node SetNodeHash
method fetches only one tile using GetSubtreeFunc, so a typical
SetMerkleNodes call results in a series of individual reads.

The only use-case for node writes in Trillian (the sequencer) does exactly one
SetMerkleNodes call per transaction with the entire write set, so there is no
need to sub-divide it into single-node writes in SubtreeCache.

This change replaces the SetNodeHash function with a batched SetNodes so
that the corresponding reads are batched too, and the single-tile callback is no
longer needed and removed.

#2378

Checklist

• I have updated the CHANGELOG.
• I have updated documentation accordingly (including the feature implementation matrix).

[pav-kv]

Review commit-by-commit might make sense.

[pav-kv]

PTAL

[pav-kv]

PTAL

[pav-kv]

Thanks Al.

@Martin2112 It would be nice to have a second pair of eyes on it too.

[pav-kv]

Addressed Martin's comments.