Added cursor closing debugging messages.
Try using thread local variables to fix the threaded cursor issues.
Added current thread to cursor closing debugging messages.
Log the current thread and connection object when parsing for a cursor.
Log cursor execution, display cursor object in cursor debug messages.
Removed cursor creation and closing debugging messages.
Set the open cursors array to nil if it becomes empty.

This will allow the empty array to be garbage collected if it is no
longer needed.
Merge branch 'thread-cursor-fix'

For https://github.com/deseretbook/ruby-plsql/pull/2