always process trie death row on commit, add more tracing

[rphmeier]

When a trie is "obliterated" by a removal -- such that it only has an empty root node remaining, the root node handle was replaced with NodeHandle::Hash(SHA3_NULL_RLP). However, the commit process would only enact deletions if the node handle type was in-memory. Future insertions could then lead to double-insertions of trie nodes into the backing database.

Also boxes branch children to reduce memory churn.

[coveralls]

Coverage decreased (-0.06%) to 85.339% when pulling c61da07 on triedbmut_double_insert into 25e6a4e on master.