Bluetooth: Merge bt_conn TX threads into a single one with k_poll

Now that the k_poll API is available we can use it to have single
connection TX thread instead of multiple ones, which helps reduce the
per-connection memory overhead by a substantial amount.

Change-Id: Icb5d4da87cf0d660bba8da43186d1e76f41c825a
Signed-off-by: Johan Hedberg <johan.hedberg@intel.com>