rdb 80 (valkey 9+) compatibility

[bpint]

In valkey 9+, the rdb format was forked, and valkey adopted new rdb version 80.

considering valkey is now widely used after redis changed its license model, we purpose this as a start of valkey rdb support.

• the rdb headers are now different, for example, "REDIS0012" and "VALKEY080"

• valkey added new opcode RDB_OPCODE_SLOT_INFO and RDB_OPCODE_SLOT_IMPORT.
  (we may have to do more someday on the later since it will have the same value of 243 as redis RDB_OPCODE_KEY_META in the future.)

• different encoding of hash with field expiration (hfe), RDB_TYPE_HASH_2.
  (Though this macro has the same value of redis RDB_TYPE_HASH_METADATA_PRE_GA, we are probably ok this time, since the later is only used in a rc version and ditched now.)
  The encoding of HASH_2 is much simpler. It just stores fields, values and expiration timestamps. However, here NO-TTL should be represented by -1 instead of 0 in rdb stream.

please review this. thank you for your advancing help.