use log_debug for 'can send now' events

fix compile

LE: require ENABLE_LE_SIGNED_WRITE for ATT/GATT Signed Writes

fix unused parameters warning

att_server: move state into hci_connection_t to handle multiple ATT clients

att_server: allow individual service implementations to register for can send now callbacks

att_server: check handle in disconnection complete event

fix warnings related to potential loss of precision for integer conversions

sm: sm_cmac_start -> sm_cmac_signed_write

att_server: drop l2cap cid for att_server_can_send_now()

att_server: drop l2cap cid for att_server_can_send_now()

att_server: use att_dispatch_server_request_can_send_now_event when needed

att_server: extract att_server_handle_can_send_now

att_server: add con handle to send related functions

att_server: fix att_can_send_now notification

don't emit X_EVENT_CAN_SEND_NOW if x_can_send_now returns false

att_server: add request can send now logic

att_server: directly process ATT Write Commands

add classic/core.h and ble/core.h, add ENABLE_CLASSIC where missing

att_server: handle l2cap_can_send_now events

event: introduce hci_event_packet_get_type

use hci_con_handle_t more

att -> att_db

bt_flip_addr(dest,src) -> reverse_bd_addr(src,dest)

introduce log_info_key and log_info_hexdump, move prototypes to btstack_debug.h