4027: API server considers arvados_sdk_version for Job reuse.

* Extend "in git" and "not in git" filters to the arvados_sdk_version
  column.
* When no filters are specified, if the submitted job specifies an
  arvados_sdk_version constraint, require reuse candidates to meet it.
* Document all this.
4027: Test updating job's SDK version after it's already set.
Merge branch '4027-sdk-constraint-reuse-wip'

Refs #4027.  Closes #4532.