Using std::string for names removes the need to handle memory
allocation. Moving the column attributes into a class of its own greatly
simplifies the creation of the TABLE_CREATE as well as modifications that
are done to it.
The HASHTABLE can be replaced with std::unordered_map. This simplifies the
management by making the deletion of old objects automatic.
More cleanup and refactoring is needed to make the contained classes
cleaner.
The instance and session objects are now C++ structs. The next pointers
for the sessions was removed as it is not the appropriate place to store
this information. This means that the client notification functionality is
broken in this commit.
