adds close() to dbs

[leo-lox]

adds a close() to the cache manager, used to enable closing the database.
Needed to close db in a bg thread

adds attach to objectbox constructor

[codecov]

Codecov Report

Attention: Patch coverage is 0% with 1 line in your changes missing coverage. Please review.

Project coverage is 72.00%. Comparing base (1551772) to head (e0eae2a).
Report is 4 commits behind head on master.

Files with missing lines	Patch %	Lines
.../repositories/cache_manager/mem_cache_manager.dart	0.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #142      +/-   ##
==========================================
- Coverage   72.01%   72.00%   -0.02%     
==========================================
  Files         113      113              
  Lines        3849     3850       +1     
==========================================
  Hits         2772     2772              
- Misses       1077     1078       +1     

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[frnandu]

looks good, thanks