Add setDatabase to Jenssegers\Mongodb\Connection

[ThomasWestrelin]

We have a use case where the database is determined dynamically and is not known in advance.
Previously, we used DB::purge() which wasn't ideal as this would cause the previous connection to be closed.

This change would allow us to change the database on demand.

[divine]

@Smolevich should we backport this?

Thanks!

[codecov-commenter]

Codecov Report

Merging #2236 (3bd58e0) into master (114afa0) will decrease coverage by 0.11%.
The diff coverage is 0.00%.

@@             Coverage Diff              @@
##             master    #2236      +/-   ##
============================================
- Coverage     86.93%   86.82%   -0.12%     
- Complexity      663      664       +1     
============================================
  Files            33       33              
  Lines          1554     1556       +2     
============================================
  Hits           1351     1351              
- Misses          203      205       +2     

Impacted Files	Coverage Δ	Complexity Δ
src/Jenssegers/Mongodb/Connection.php	88.73% <0.00%> (-2.58%)	39.00 <1.00> (+1.00)	⬇️

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