Don't call std::terminate on connection drop

[valbok]

https://bugs.mysql.com/bug.php?id=116569

If there is connection drop during executing the query, an exception is thrown from cdk::mysqlx::Cursor::~Cursor() or cdk::mysqlx::Stmt_op::~Stmt_op().

note: in C++11 destructors default to ‘noexcept’ - that means it leads to call std::terminate().

$ sudo XPLUGIN_PORT=5445  ./run_unit_tests --gtest_filter=Sess.connection_drop
[ RUN      ] Sess.connection_drop
terminate called after throwing an instance of 'cdk::foundation::Error'
  what():  CDK Error: OpenSSL: error:0A000126:SSL routines::unexpected eof while reading
Aborted

NOTE: Sess.connection_drop test case may should be excluded since it uses tc + system

There might be several ways to reproduce the terminate.

1. Kill MySQL during the execution of SELECT query: killall -9 mysqld
2. Forcing 100% packet loss and after mysqlx_write_timeout seconds MySQL will close the connection.

Using traffic control to force the packet loss + mysqlx_write_timeout can also introduce the infinite hang in TLS::Read_some_op::common_read() or recv_some()->poll_one() , similar to https://bugs.mysql.com/bug.php?id=92325

This MR introduces suggested fix to fix the terminate when the connection is closed.

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