Bluetooth: Rename bt_conn_get/put to bt_conn_ref/unref

We want to make the naming convention ref/unref rather than get/put.
So far the only reference counted objects are the buffers and the
connections. For the buffers the new generic buffer API will also use
ref/unref.

Change-Id: I9fe8b8a6a50a8baf06ba231e8f6717a5a47dd292
Signed-off-by: Johan Hedberg <johan.hedberg@intel.com>