Commit 206955e4 "streaming: Reduce memory usage when sending mutations"
moved streaming mutation limiter from do_send_mutations() to
send_mutations(). The reason for that was that send_mutation() did full
mutation copies. That's no longer the case and streaming limiter should
be moved back to do_send_mutation() in order to provide back pressure to
fragment_and_freeze().
Signed-off-by: Paweł Dziepak <pdziepak@scylladb.com>