Merge single and bulk endpoints

[nwneisen]

FEATURE REQUEST

1. Is there an open issue addressing this request? If it does, please add a "+1" reaction to the
   existing issue, otherwise proceed to step 2.
   No

2. Describe the feature you are requesting, as well as the possible use case(s) for it.
   The single creation endpoints for things (/things), channels (/channels), and connections (/channels/:chIDs/things/:thingIDs) should be removed in favor of the bulk creation endpoints (/things/bulk, /channels/bulk, /connect). The bulk creation endpoints should be changed over to use /things and /channels. Docs should also be updated to no longer include information about the single endpoints. Initially discussed in MF-906 - Change single creation endpoints to use bulk service calls #927 and Add bulk connect #898. This change should not be done until v1.0.0.

3. Indicate the importance of this feature to you (must-have, should-have, nice-to-have).
   must-have