Database Model: minor clean up

[jonas-rem]

Based on ~#27~, #29,needs rebase after those two PRs have been merged.

Resource Table:

• [x] remove bool_type for resource, used rarely and we can use int_type instead (0|1)

Device Table:

• [x] rename device_id to endpoint which is the official LwM2M term, leads to less confusion
• [x] remove redundand name entry. Devices do not have a name, instead the endpoint can be considered the unique name of a device.