See #6160.

Fixes issue #6418.

Fixes issue #6418.

Streams items are similar to dictionaries, however they preserve both
the order, and allow for duplicated field names. So a map is not a
semantically sounding way to deal with this.

https://twitter.com/antirez/status/1248261087553880069

Related to #3243.

Reloading of the RDB generated by

    DEBUG POPULATE 5000000
    SAVE

is now 25% faster.

This commit also prepares the ability to have more flexibility when
loading stuff from the RDB, since we no longer use dbAdd() but can
control exactly how things are added in the database.

See #7071.

Related to #5145.

Design note: clients may change type when they turn into replicas or are
moved into the Pub/Sub category and so forth. Moreover the recomputation
of the bytes used is problematic for obvious reasons: it changes
continuously, so as a conservative way to avoid accumulating errors,
each client remembers the contribution it gave to the sum, and removes
it when it is freed or before updating it with the new memory usage.

fix comments about RESIZE DB opcode in rdb.c

Delete keys in async way when executing DEL command, if
lazyfree-lazy-user-del is yes.

Initially they needed to be at the end so that we could extend to N
strings in the future, but after further consideration I no longer
believe it's worth it.

Checking OOM by `getMaxMemoryState` inside script might get different result
with `freeMemoryIfNeededAndSafe` at script start, because lua stack and
arguments also consume memory.

This leads to memory `borderline` when memory grows near server.maxmemory:

- `freeMemoryIfNeededAndSafe` at script start detects no OOM, no memory freed
- `getMaxMemoryState` inside script detects OOM, script aborted

We solve this 'borderline' issue by saving OOM state at script start to get
stable lua OOM state.

related to issue #6565 and #5250.