[2.x] Don't purge central connections

[stancl]

Fixes #176

The endTenancy() method (indirectly) calls DatabaseManager::reconnect() which reconnects the default database back to the central database. That method purges the connection that we are switching to - this makes sense in case of switching tenants, to purge the 'tenant' connection, but in case of ending tenancy, this purges the central connection, resulting in DatabaseStorageDriver::$centralDatabase->getPdo() returning null, making it unable to delete tenants after tenancy was ended (such as in tests' tearDown()).

TODO:

• Regression test

[codecov]

Codecov Report

Merging #189 into 2.x will increase coverage by 0.03%.
The diff coverage is 100%.

@@             Coverage Diff             @@
##                2.x    #189      +/-   ##
===========================================
+ Coverage     89.06%   89.1%   +0.03%     
- Complexity      315     316       +1     
===========================================
  Files            52      52              
  Lines           924     927       +3     
===========================================
+ Hits            823     826       +3     
  Misses          101     101

Impacted Files	Coverage Δ	Complexity Δ
src/DatabaseManager.php	94.11% <100%> (+0.36%)	20 <1> (+1)	⬆️

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