Thread Pool Maintenance and Bugfix

Fixed a typo in a comment in thread_pool.hpp.

Cleaned up the worker thread wait/wake code, and fixed a bug therein that was causing scheduled tasks to be ignored indefinitely in thread_pool.cpp.
World Update -- Maintenance

Upon installation the world module will now begin a maintenance loop.  At an interval which may be configured (but which defaults to once every 5 minutes) all columns will be scanned, saved (if they are "dirty"), and unloaded (if they have no attached players and their interest count is zero).

Factored the code for saving columns out of the maintenance function, and changed the way in which columns are saved: They are copied onto the stack and saved from there, which drastically cuts down on the amount of time a lock is held on the column to save it (since the lock is only held long enough for a quick copy, rather than long enough for a write to the backing store).

Implemented better/more verbose error handling in the process function.

Implemented logging in the set_seed function.

Added code to load WorldContainer settings from the backing store upon installation.

Made various tweaks to the ColumnContainer class to support new/improved/changed functionality.

Columns will no longer be "dirty" after simply being loaded from the backing store.
ThreadPool Fix

Fixed a bug which caused scheduled tasks to be inserted in the wrong order, causing tasks delayed for a short period of time to be delayed for a much longer period of time.
WorldContainer Maintenance Change

Moved the code which continues the maintenance loop inside the try block so that if scheduling the next iteration fails it is logged and causes a server panic.
WorldContainer Interest

Callers may now express interest in WorldContainer columns, causing those columns to be loaded/populated/generated as applicable and to remain loaded until interest is released.

This is necessary so that the spawn area may be generated when the server starts up, rather than when the first player logs in (potentially incurring a long delay).