directory: zdbfs seems to crash when adding 10 millions file in a directory

[maxux]

Reported by threefoldtech/quantum-storage#24

It seems that zdbfs crashes when adding 10 millions files. I'll investigate, could indirection list be full but it should not crash and report an error.

[maxux]

Seems to be a out-of-memory issue and not zdbfs issue. That said, that show some huge memory usage because of the cache (there is no limit set). This should maybe have some think about.