The responses from node RPCs encode hash values as hexadecimal strings. This
behaviour is stipulated in our OpenAPI documentation. In some cases, however,
hashes received as JSON parameters were being decoded as byte buffers, as is
the convention for JSON.
This resulted in the confusing situation that a hash reported by one request
(e.g., broadcast_tx_commit) could not be passed as a parameter to another
(e.g., tx) via JSON, without translating the hex-encoded output hash into the
base64 encoding used by JSON for opaque bytes.
Fixes#6802.