replace list and ringbuffer with fixed queue

use eventgroup to sync spp_vfs_write

each connection has a switch_delay_timer not sharing a one

revert functions like spp_find_slot_by_xxx

fix vfs read bug when peer close

components/bt/host/bluedroid/stack/include/stack/btu.h

@@ -290,6 +290,8 @@ UINT16 BTU_BleAclPktSize(void);
 
 bool btu_task_post(uint32_t sig, void *param, uint32_t timeout);
 
+int get_btu_work_queue_size(void);
+
 /*
 #ifdef __cplusplus
 }