Multi-threading issues

[ValdikSS]

There are several race conditions which results in printing less pages then the original raster file has.

[ValdikSS]

Fixed one exact issue which affected me, but there's lots of places which modify data without proper locking.

The whole caching subsystem is unnecessary complex: it not only processes the pages way ahead of the printing, but also could swap them to disk if more than 30 pages have been processed in the background but not printed yet!
And swapping is done in /tmp, which is tmpfs (RAM) on the modern systems.