Various Espressif BLE fixes

[dhalbert]

• Fixes Espressif BLE: iBBQ not working #9361
• Fixes Espressif BLE: Heart Rate not working #9338
• Fixes Espressif BLE: uart_echo_client not working: regression from 9.0.5 #9337

Changes:

• In Adapter.c and Connection.c, busy-wait for incoming events using status variables, instead of using FreeRTOS task notification. This fixes some breaking out-of-order notifications I saw during GATT discovery, and and also allows for background tasks to run and also allows for ctrl-C interruption.
• Characteristic observer when not present should have been mp_const_none, not NULL. This fixed a hard crash when there was no observer.
• Use GATT_MAX_DATA_LENGTH, not 0, for default size of remote characteristics. When zero, the initial PacketBuffer ringbuf was set too small.
• Retry a few times after occasional HS_BLE_EBUSY when starting a GAP scan.
• A little cleanup here and there.

Tested with:

• ble_ibbq_simpletest.py - iBBQ NutriChef thermometer
• ble_heart_rate_simpletest.py - Schosche heart rate monitor
• ble_uart_echo_test.py and ble_uart_echo_client.py - tested both directions, talking to an nRF52840.

@jedgarpark This should fix your iBBQ project issue.
@tyeth Does this solve #9304 for you?

[tyeth]

Lovely, thanks for the work and ping, I'll probably get to it Thursday

[tannewt]

Looks good! Thank you for the fixes!