MB-10957: Send hello command when restoring

Since we restore 3.0 data using memcached protocol, we need to
let server know that we handle datatype field.

However, we should NOT send hello comand when restorign 2.x data and
leave it for server to process it.

Change-Id: Ifea875f1190076302b6b9f246904b615d7dc8bd8
Reviewed-on: http://review.couchbase.org/36579
Reviewed-by: Steve Yen <steve.yen@gmail.com>
Tested-by: Bin Cui <bin.cui@gmail.com>