api/txdb: insert asset definitions faster
api/txdb: insert asset definitions faster

Three changes:

1. Don't bother hashing (or inserting) an empty definition.
2. Don't update existing definitions (they always have
the same data).
3. Insert the whole block all in one batch.

Closes chain/chainprv#412.
Reviewers: da39a3ee5e6b4b0d3255bfef95601890afd80709@erykwalder

chain/chain,chain/chain,chain/chain,chain/chain,chain/chain,chain/chain