Launching the WebAPI service

This is how the back-end's webapi service should be launched during startup phase:

1. if webapi service is running on configured port:
   • if webapi version is compatible: success, else exit with error
2. if webapi executable is configured:
   1. if executable does not exists: exit with error
   2. start webapi executable. if not possible: exit with error
   3. get webapi version
   4. if webapi version is compatible: success, else exit with error
3. collect installed and valid cate-core instances (from ~/.cate/versions)
   1. sort by version number difference in ascending order. For each cate-core instance:
      1. start webapi executable. if not possible: continue
      2. get webapi version
      3. if webapi version is compatible: success, else continue
   2. exit with error "No compatible back-end found."