Interchange: HOLD_WORKER command should not parse its own private protocol

[benclifford]

Is your feature request related to a problem? Please describe.
Interchange commands and responses are sent in structured objects (using pickle).

Nevertheless, the HOLD_WORKERS command contains an ad-hoc UTF-8/ASCII protocol to pass its parameter, which is treated as a byte string elsewhere in htex, not UTF-8:

2024-04-20 08:36:13.204 interchange:282 HTEX-Interchange(131272) Interchange-Command _command_server [DEBUG] Received command request: HOLD_WORKER;d9052d12878b

This command should use the existing data structures available for conveying a separate command and byte sequence argument, such as a tagged dictionary {"type": "hold_worker", "manager_id", bytes(d9052d12878b)} or a tuple.

This would remove some construction/parsing complexity and potentially give some cross-network type safety.

[benclifford]

crossref #3839 for another HOLD_WORKER tidyup

[benclifford]

see more protocol structure comment here #3840 (comment)