Fix crash when Pool::Entry::disconnect() is called

[valbok]

Many Pool::Entry objects can keep the same pointer to Pool::Connection. If Pool::Entry::disconnect() is called on one such object, Pool::removeConnection() is called to remove Pool::Connection from the pool, where connection->ref_count is cleared and connection->removed_from_pool is set.

Next Pool::Entry::~Entry() calls decrementRefCount() with

1. const auto ref_count = data->ref_count.fetch_sub(1); where data->ref_count will be negative, since it was cleared
2. checks removed_from_pool and deletes Pool::Connection but there might be multiple Entry objects still keep pointer to this Pool::Connection

Suggesting not to clear ref_count on disconnect()
and delete Pool::Connection only on the last Pool::Entry is being destroyed.

Fixes ea375ef

Changelog category (leave one):

• Bug Fix (user-visible misbehavior in an official stable release)

Changelog entry (a user-readable short description of the changes that goes to CHANGELOG.md):

• Fixed crash when Pool::Entry::disconnect() is called

Information about CI checks: https://clickhouse.com/docs/en/development/continuous-integration/

[CLAassistant]

All committers have signed the CLA.

[robot-ch-test-poll2]

This is an automated comment for commit ecea1ac with description of existing statuses. It's updated for the latest CI running
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