Bluetooth: att: use a dedicated metadata struct for RSP PDUs

The ATT module has provisions to queue a packet/buffer for sending later if
it can't send it right away. For example if the conn.c tx context
allocation fails.

This unfortunately doesn't work if the buffer can't get allocated in the
first place, or if the ATT metadata can't also be allocated.

The metadata is allocated from a global pool set to the same number as
conn.c TX contexts. That can lead to a situation where other users of ATT
manage to queue a bunch of buffers (e.g. the app spamming GATT
notifications), depleting the number of ATT metadata slots so that none are
available.

When none are available, and we receive an ATT REQ, we try to allocate one,
fail, and drop the buffer (!). That pretty much guarantees an ATT timeout.

As a workaround for this, use a per-channel metadata slot, that is only
used for completing transactions.

Signed-off-by: Jonathan Rico <jonathan.rico@nordicsemi.no>