[6.1] Fix 3640 | Remove extra connection deactivation.

[mdaigle]

Description

When I originally implemented #3019, due to a misunderstanding about when connections are activated/deactivated, I added an extraneous call to DbConnectionInternal.DeactivateConnection as part of the PutObjectFromTransactedPool. Connections are always deactivated at the time they are returned to the pool via the ReturnInternalConnection method and do not need to be activated/deactivated as they move between the main pool and the transacted pool. The activate and deactivate methods modify a counter value that tracks the number of connections in active use. This extra deactivate led to active connection counts going negative.

However, removing this DeactivateConnection call is also not entirely correct. Part of the deactivation process is setting the "reset" flags via a call to the ResetConnection method. These flags determine the Status value sent to the server when a connection is recycled out of the pool. Connections that are placed in the transacted pool may use status RESETCONNECTIONSKIPTRAN if they are participating in a distributed transaction. This gives extra guarantees that the server will not reset the connection's transaction state. When moving a connection out of the transacted pool and back into the main pool, we no longer want the server to maintain the association to the transaction because the transaction has ended and we want to use the connection for general purpose commands. Therefore, we need to downgrade the connection's status value to RESETCONNECTION. This way, the next time the connection is used its association with the transaction will be removed.

To appropriately set the reset flags without impacting the active connection counter, I exposed the ResetConnection method as internal so that it can be called directly by the pool. All other actions taken in DeactivateConnection are safe to skip at this point.

I hope to clean up this flow as part of adding transaction support to the new pool implementation, but it's out of scope for this PR.

Issues

#3640

Testing

None of our tests cover metrics. Working on adding that as part of the new connection pool work.
In the meantime, this change will be verified manually.

Existing distributed transaction tests show that the reset flags are still reevaluated correctly.

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[codecov]

Codecov Report

❌ Patch coverage is 50.00000% with 1 line in your changes missing coverage. Please review.
✅ Project coverage is 65.86%. Comparing base (5200419) to head (488d937).
⚠️ Report is 3 commits behind head on release/6.1.

Files with missing lines	Patch %	Lines
.../Microsoft/Data/ProviderBase/DbConnectionClosed.cs	0.00%	1 Missing ⚠️

Additional details and impacted files

@@               Coverage Diff               @@
##           release/6.1    #3776      +/-   ##
===============================================
- Coverage        66.69%   65.86%   -0.83%     
===============================================
  Files              279      279              
  Lines            61764    61765       +1     
===============================================
- Hits             41192    40681     -511     
- Misses           20572    21084     +512     

Flag	Coverage Δ
addons	90.82% <ø> (ø)
netcore	68.56% <50.00%> (+0.02%)	⬆️
netfx	68.63% <50.00%> (-1.16%)	⬇️

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