Replace builtin json to ffjson for incoming transaction

> We already discussed data serialization in #434

JSON is wide spreadex and well known format, so for end user, especially client-side we should support JSON for incoming transaction.

There are so many critics against the performance of Golang's builtin JSON, we can adaopt the ffjson for performance.

No files or tests are identified in the issue. Start by locating the incoming transaction JSON serialization and review the related discussion in issue #434; completion should be defined by the project’s tests and by confirming that the selected serialization path uses ffjson without changing the transaction format.

Written by the indexing model from the issue text.