Web bluetooth can't subscribe/notify to characteristics

[agrath]

Starting zwack to announce a RSC service, and connecting to the service and attempting to subscribe to the notify of the measurement charactersitic fails with a GATT unknown error in Chrome.

See: https://stackoverflow.com/questions/65290098/ble-browser-api-startnotifications-returns-gatt-error-not-supported/79344297#79344297 (with my answer)

Ultimately, the root cause is upstream in abandonware@bleno https://github.com/abandonware/bleno/blob/master/lib/hci-socket/gatt.js#L208 where the 2902 characteristic descriptor is added automatically if the characteristic is marked as notify.

I have created an issue there abandonware/bleno#51, and a PR abandonware/bleno#52

However, in Zwack, these descriptors are also declared manually on the service/characteristic, for example: https://github.com/paixaop/zwack/blob/master/lib/rsc/rsc-measurement-characteristic.js#L22
This results in a duplicate 2902 descriptor 'on the air' which causes Chrome to be unable to subscribe.

PR incoming