Store and read Redis hashes for objects using HSET/HGET/HGETALL (redis-cli and clients)

Documented steps

1. Create/update hash fields: `HSET myhash field1 "Hello"` (creates the hash if absent).
2. Set multiple fields in one command: `HSET myhash field2 "Hi" field3 "World"` (recommended for efficiency - fewer round trips).
3. Read one field: `HGET myhash field1`; missing field returns nil.
4. Read all fields/values: `HGETALL myhash` (returns flat field,value,field,value list in RESP2; a map in RESP3).
5. Read several specific fields: `HMGET myhash field1 field2`; get count with `HLEN myhash`.
6. HSET returns the number of NEW fields added (not overwritten ones) - useful to detect whether a field already existed.
7. python: `r.hset('h', mapping={...})`, `r.hgetall('h')`; node-redis: `client.hSet('h', {...})`, `client.hGetAll('h')`.