Refresh pickup markers more often

See: #901 (closed)

What does this MR do?

This allows refreshing pickup markers on different actions and save it in $_SESSION.

• after every pickup slot join/leave/kick action (page reload necessary)
• after a timeout of x minutes on page reload without an extra sql or redis query
• refresh rate can now be easily set per user level. Ambassadors or store managers can have a short timespan while the large amount of foodsavers could have a rather high timespan. I think we should discuss numbers shortly before merge

~"help wanted" with:

• changing and fitting our large query maybe from ?@dthulke
  • so it loads all stores except filtered ones with these fields
  • list store name
  • lists them only for $fsId
• we need to think about if we want to keep parts of the old implementation to have some flexibility to slowly enable this feature for different user groups. Or discuss if we need this flexibility. We could also control this with longer refresh rates like mentioned above

How confident are you it won't break things if deployed?

Should work

Links to related issues

• Latest Slack pickup marker strategy discussion: https://yunity.slack.com/archives/C1T1ZM9A4/p1572703338028400

How to test

Steps a reviewer can take to verify that this MR does what it says it does e.g.
(depends on the final implementation)

Screenshots (if applicable)

None - backend change.

Checklist

• added a test, or explain why one is not needed/possible...
• no unrelated changes
• asked someone for a code review
• joined #foodsharing-beta channel at https://slackin.yunity.org
• added an entry to CHANGELOG.md (description, merge request link, username(s))
• Once your MR has been merged, you are responsible to update the #foodsharing-beta Slack channel about what has been changed here. They will test your work in different browsers, roles or other settings