Background garbage collection

Superserver has, even since before version 1.0, performed background garbage collection where the server informs the garbage collector about old versions of updated and deleted rows when they are ready to be cleaned up. This helps avoid the need to force a full scan of each record in the database tables to get the garbage collector to remove these old versions.

When all rows in a table are read by the server, any old record versions are flagged to the garbage collector as being ready to be cleared out. They are not deleted by the scanning process as in the cooperative method. The garbage collector runs as a separate background thread and it will, at some point, remove these older record versions from the database.