Move common modules to datamodel, and rename datamodel accordingly

[sajith]

Both sdx-controller and sdx-lc has some duplicated code related to DB handling and messaging:

• https://github.com/atlanticwave-sdx/sdx-controller/tree/main/swagger_server/messaging
• https://github.com/atlanticwave-sdx/sdx-lc/tree/main/swagger_server/messaging
• https://github.com/atlanticwave-sdx/sdx-controller/blob/main/swagger_server/utils/db_utils.py
• https://github.com/atlanticwave-sdx/sdx-lc/blob/main/swagger_server/utils/db_utils.py

They need better test coverage as well. It would be good to avoid that duplication, and move them somewhere common, and add some tests. These are the options we considered in a discussion:

1. Refactor the common code into a new "sdx-base" library. Downside is that we'll have to manage yet another repository.
2. Have sdx-lc use the modules from sdx-controller. That will probably make sdx-controller a libary, which probably is an okay trade-off. We get to develop and test these common modules in one place. We can use the same MQ and DB instances used by sdx-controller when developing and testing these modules.
3. Move the modules here in datamodel, and then rename "datamodel" to "sdx-base", to reflect the fact that it is not just datamodels anymore. But then we will need to use an MQ and a DB when adding integration tests.

Of these, option 1 is the least favored one. Option 3 appears to be a little more preferable than option 2, but option 3 is also a little inconvenient.