Fix recheck on processes which are already grouped

[frazew]

-recheck already allows process_exporter to put processes which might change name after they start in the right group. However this only works if the initial process name is not matched. In case a matcher catches it early, the process is left in its old group after its name changes.

This can be observed with a generic configuration like this:

process_names:
  - name: "{{.Comm}}"
    cmdline:
      - '.+'

If a process called process1 with PID 1234 starts and later execs and changes its name to process2, then 1234 remains in the process1 group even though it should now be in a new process2 group.

This PR removes the assumption that GetStatic is static, and instead retrieves and updates it at each scrape. I believe this should not have any noticeable performance hit?

• if recheck (or recheck-with-time-limit) is enabled, the process is allowed to change groups after it has started
• if the new process name is no longer matched by the namer, the process is ignored I'm not sure about that last behavior: is it Update the right place to do it? Could it break tracking of child processes?