nimble/host: Optimize characteristics discovery

We can stop characteristics discovery as soon as there are less than 2
attributes left. This means if last characteristic has descriptor (e.g.
CCC) we will save single ATT_READ_BY_TYPE_REQ/RSP transaction.

As a side-effect we also resume discovery starting from the next handle
after last characteristic value handle instead of characteristic
declaration handle, but this is ok since there cannot be any atrtibutes
(so also characteristic declarations) between characteristic declaration
and characteristic value.

nimble/host/src/ble_gattc.c

@@ -2299,7 +2299,9 @@ ble_gattc_disc_all_chrs_rx_adata(struct ble_gattc_proc *proc,
         rc = BLE_HS_EBADDATA;
         goto done;
     }
-    proc->disc_all_chrs.prev_handle = adata->att_handle;
+
+    /* We'll resume after characteristic value */
+    proc->disc_all_chrs.prev_handle = chr.val_handle;
 
     rc = 0;
 
@@ -2328,7 +2330,10 @@ ble_gattc_disc_all_chrs_rx_complete(struct ble_gattc_proc *proc, int status)
         return BLE_HS_EDONE;
     }
 
-    if (proc->disc_all_chrs.prev_handle == proc->disc_all_chrs.end_handle) {
+    /* We can stop discovery if there are less than 2 attributes left since
+     * complete characteristic requires at least 2 handles.
+     */
+    if (proc->disc_all_chrs.prev_handle + 1 >= proc->disc_all_chrs.end_handle - 1) {
         /* Characteristic discovery complete. */
         ble_gattc_disc_all_chrs_cb(proc, BLE_HS_EDONE, 0, NULL);
         return BLE_HS_EDONE;