Node fixes for debug mode

_From @theoreticalbts on March 1, 2016 15:35_

Debug mode #596 has revealed many race conditions in `node.cpp` close() function.

This ticket is to keep track of the fixes for those issues

_Copied from original issue: cryptonomex/graphene#604_

Start with node.cpp and the close() function, then read the debug mode context in issue #596 to understand the reported race conditions. Trace each failure reproduced there and document or verify fixes for all affected close() paths; the issue does not name tests or a complete done condition.

Written by the indexing model from the issue text.