Fix connection_wrapper destructor sometimes closing other connections

[meastham]

Currently, if a dbapi connection is closed using connection.close(), and then another dbapi connection is opened before the first connection is destroyed, the destruction of the first connection's python object will cause the second connection to be closed.

This happens because connection_wrapper.conn continues to point at the global chdb_conn* after close() runs, so it's possible for the connection_wrapper destructor to close a connection it didn't create.

I used this short script to test this:

conn1 = dbapi.connect("/tmp/chdb1")
cur1 = conn1.cursor()
cur1.execute('select version()')
cur1.close()
conn1.close()

conn2 = dbapi.connect("/tmp/chdb2")
cur2 = conn2.cursor()

# Works
cur2.execute('select version()')
print(cur2.fetchone())

# Cause the closed connection to be destroyed
del cur1
del conn1

# Exception: Invalid or closed connection
cur2.execute('select version()')
print(cur2.fetchone())

the second to last line throws # Exception: Invalid or closed connection with chdb 3.1.1, but not with a wheel built from this commit.

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

Fix connection_wrapper destructor sometimes closing other connections

Documentation entry for user-facing changes

• Documentation is written (mandatory for new features)

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[meastham]

Had to force-push to fix the author on my commit so the CLA check would pass, but the content is all the same