OSD::handle_misdirected_op: handle ops to the wrong shard

OSD recomputes op target based on current OSDMap. With an EC pg, we can get
this result:
1) client at map 512 sends an op to osd 3, pg_t 3.9 based on mapping
   [CRUSH_ITEM_NONE, 2, 3]/3
2) OSD 3 at map 513 remaps op to osd 3, spg_t 3.9s0 based on mapping [3, 2, 3]/3
3) PG 3.9s0 dequeues the op at epoch 512 and notices that it isn't
   primary -- misdirected op
4) client resends and this time PG 3.9s0 having caught up to 513 gets it and
   fulfils it

We can't compute the op target based on the sending map epoch due to
splitting.  The simplest thing is to detect such cases in
OSD::handle_misdirected_op and drop them without an error (the client
will resend anyway).

Signed-off-by: Samuel Just <sam.just@inktank.com>
ECBackend: don't leak transactions

Fixes: #7539
Signed-off-by: Samuel Just <sam.just@inktank.com>