fix: add store service cleanup

[vanmxpx]

PR Checklist

Please check if your PR fulfills the following requirements:

• The commit message follows our guidelines: https://github.com/ngxs/store/blob/master/CONTRIBUTING.md#commit
• Tests for the changes have been added (for bug fixes / features)
• Docs have been added / updated (for bug fixes / features)

PR Type

What kind of change does this PR introduce?

[x] Bugfix
[ ] Feature
[ ] Code style update (formatting, local variables)
[ ] Refactoring (no functional changes, no api changes)
[ ] Build related changes
[ ] CI related changes
[ ] Documentation content changes
[ ] Other... Please describe:

What is the current behavior?

Reference to Store service object keeps after the module is destroyed.

Issue Number: #116

What is the new behavior?

Reference to Store service object clears after the module is destroyed.
(same behavior as with "injector" reference)

Does this PR introduce a breaking change?

[ ] Yes
[x] No

Other information

[XavierDupessey]

Not sure who owns this project and could take a look, maybe @arturovt?

[arturovt]

Needs to be rebased on master. Also where the store is being set? I see it's initialized and set to null in both places.

[vanmxpx]

Needs to be rebased on master. Also where the store is being set? I see it's initialized and set to null in both places.

@arturovt You're right I've lost initialization

sorry but I broke the branch rebasing it to upstream so I'm forced to reopen pull request with all fixes here #154

this one can be discarded