Cache statistics

[xmas79]

Branched from 1.4.1 tag.

I implemented some stats for the upper filesystem block usage. For the entire filesystem (Yes! it will consume a lot of memory, 36 bytes per block in this implementation) we now track how many times each block has been read, and how many time it has been written. We also track the "cumulative" time between reads and writes, so in the output we can average them and gather some usage patterns. I added also "collisions" and "replaces" in the hashtable implmementations.

TODO: Dump the usage stats (only used blocks just to shorten the output) and the hash collisions stats (better in another FUSE file?)