Mock file system subsystems to test disk failures

The "Files are hard" essay provides an overview of (not so) current state of disk/system crash resilence for different databases and file systems.

Please note that only SQLite passed every test.

The test harness for SQLite uses step count in the special testing file system support subsystem. After specified I/O count operations that subsystem implementation simulates a crash - disk or other failure, - with the damage spread to some data around the last operation.

We have IDBDataFile.(h, cpp) source code which provide interface to abstract file system functions. This can be used akin to SQLite's testing file system plugin to test crash resilience.