Tracking of the node where the last COM_STMT_EXECUTE was sent allows
routing of all following COM_STMT_FETCH to the same node. This is required
for cursors to work.
MariaDB/MySQL does not support multiple active cursors so the
COM_STMT_FETCH will always refer to the latest COM_STMT_EXECUTE and using
a different ID goes against the protocol. If/when the support for multiple
active cursors is added, the tracking should be done for each
COM_STMT_EXECUTE statement. This should be relatively easily to achieve
but currently it is unnecessary.
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