Bluetooth: host: Update GATT to use new delayable work API

Update GATT to use the new delayable work API for the delayed
calculation of the database hash and the service changed indication.

When the database hash characteristic is read the hash work needs to be
canceled if in progress and if currently executing wait for it to
complete so that the threads don't both write to the stored value.

Signed-off-by: Joakim Andersson <joakim.andersson@nordicsemi.no>