make sqlfs_destroy() to clean up after sqlfs_init()

sqlfs_init() is required for both "init" mode and "open/close" mode to set
up things like the path to the file and the key to the database.
sqlfs_open() calls sqlfs_init() to do that.  In order to leave things fully
clean, there needs to be also a sqlfs_destroy(), this is then also called
by sqlfs_close().  In threaded/init mode, sqlfs_init() must be called
before any threads start operating, and sqlfs_destroy() must be called
after all threads complete operation.
ignore all test db files
reorganize test after init vs open/close operating modes