1. start an esp_timer on entering sleep to acquire pm_lock before wake up from modem sleep
2. decrease the clock division of XTAL fed to bluetooth low power clock from 32us to 2us period to allow to work under 240MHz Max CPU frequency
3. decrease the minimum sleep duration threshold to allow shorter bluetooth modem sleep period, especially for BLE with short connection interval
4. reconfigure bluetooth baseband(BT-BB) settings after PHY/RF init upon waking up from modem sleep to avoid packet RX/TX performance degradation
1. provide options for bluetooth low power mode
2. provide two options for bluetooth low power clock: main XTAL and external 32kHz XTAL
3. provide function and callbacks to control bluetooth low power mode, including enable/disable sleep, software wakeup request, low power clock settings, check power state, etc
4. modify vhci API vhci_host_send_packet to use blocking mode
5. note that DFS and bluetooth modem sleep can not be used together currently.
1. fix the bug. Modify the condition that esp_bt_controller_mem_release() shoud be only called before esp_bt_controller_init() or after esp_bt_controller_deinit()
2. modify the example to use esp_bt_controller_mem_release()
