Fix issue with close and MongoStore and update _collection attribute

[munrojm]

This PR fixes an issue where collection was not getting set back to None when closing the MongoDB connection. This is required to support changes in pymongo==4.0.

Also, collection as store property is replaced with _collection to provide better error handling surrounding connection issues.

[codecov]

Codecov Report

Merging #523 (d0e4b9e) into main (0f384dc) will increase coverage by 0.02%.
The diff coverage is 91.83%.

@@            Coverage Diff             @@
##             main     #523      +/-   ##
==========================================
+ Coverage   88.97%   88.99%   +0.02%     
==========================================
  Files          40       40              
  Lines        2738     2735       -3     
==========================================
- Hits         2436     2434       -2     
+ Misses        302      301       -1     

Impacted Files	Coverage Δ
src/maggma/stores/aws.py	87.95% <66.66%> (-0.13%)	⬇️
src/maggma/stores/advanced_stores.py	78.72% <80.00%> (ø)
src/maggma/stores/compound_stores.py	81.56% <90.00%> (-0.11%)	⬇️
src/maggma/stores/gridfs.py	94.25% <90.00%> (ø)
src/maggma/core/store.py	86.13% <100.00%> (-0.14%)	⬇️
src/maggma/stores/mongolike.py	86.64% <100.00%> (+0.33%)	⬆️

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update baff06b...d0e4b9e. Read the comment docs.

[lgtm-com]

This pull request introduces 4 alerts when merging 01d9ce4 into baff06b - view on LGTM.com

new alerts:

• 4 for Unused import