Atomically transfer a key from a source Redis instance to a destination Redis
instance. On success the key is deleted from the original instance and is
guaranteed to exist in the target instance.

The command is atomic and blocks the two instances for the time required to
transfer the key, at any given time the key will appear to exist in a given
instance or in the other instance, unless a timeout error occurs. In 3.2 and
above, multiple keys can be pipelined in a single call to `MIGRATE` by passing
the empty string ("") as key and adding the `KEYS` clause.

The command internally uses `DUMP` to generate the serialized version of the key
value, and `RESTORE` in order to synthesize the key in the target instance. The
source instance acts as a client for the target instance. If the target instance
returns OK to the `RESTORE` command, the source instance deletes the key using
`DEL`.

The timeout specifies the maximum idle time in any moment of the communication
with the destination instance in milliseconds. This means that the operation
does not need to be completed within the specified amount of milliseconds, but
that the transfer should make progresses without blocking for more than the
specified amount of milliseconds.

`MIGRATE` needs to perform I/O operations and to honor the specified timeout.
When there is an I/O error during the transfer or if the timeout is reached the
operation is aborted and the special error - `IOERR` returned. When this happens
the following two cases are possible:

- The key may be on both the instances.
- The key may be only in the source instance.

It is not possible for the key to get lost in the event of a timeout, but the
client calling `MIGRATE`, in the event of a timeout error, should check if the
key is _also_ present in the target instance and act accordingly.

When any other error is returned (starting with `ERR`) `MIGRATE` guarantees that
the key is still only present in the originating instance (unless a key with the
same name was also _already_ present on the target instance).

If there are no keys to migrate in the source instance `NOKEY` is returned.
Because missing keys are possible in normal conditions, from expiry for example,
`NOKEY` isn't an error.

## Migrating multiple keys with a single command call

Starting with Redis 3.0.6 `MIGRATE` supports a new bulk-migration mode that uses
pipelining in order to migrate multiple keys between instances without incurring
in the round trip time latency and other overheads that there are when moving
each key with a single `MIGRATE` call.

In order to enable this form, the `KEYS` option is used, and the normal _key_
argument is set to an empty string. The actual key names will be provided after
the `KEYS` argument itself, like in the following example:

    MIGRATE 192.168.1.34 6379 "" 0 5000 KEYS key1 key2 key3

When this form is used the `NOKEY` status code is only returned when none of the
keys is present in the instance, otherwise the command is executed, even if just
a single key exists.

## Options

- `COPY` -- Do not remove the key from the local instance.
- `REPLACE` -- Replace existing key on the remote instance.
- `KEYS` -- If the key argument is an empty string, the command will instead
  migrate all the keys that follow the `KEYS` option (see the above section for
  more info).
- `AUTH` -- Authenticate with the given password to the remote instance.
- `AUTH2` -- Authenticate with the given username and password pair (Redis 6 or
  greater ACL auth style).

`COPY` and `REPLACE` are available only in 3.0 and above. `KEYS` is available
starting with Redis 3.0.6. `AUTH` is available starting with Redis 4.0.7.
`AUTH2` is available starting with Redis 6.0.0.

@return

@simple-string-reply: The command returns OK on success, or `NOKEY` if no keys
were found in the source instance.