For WCFS to be efficient it will have to carefully preserve OS cache on
file invalidations. As preparatory step establish infrastructure for
verifying state of OS file cache and start asserting on OS cache state
in a couple of places.

See comments added to tFile constructor that describe how OS cache state
verification is setup.

Some preliminary history:

8293025b    X Thoughts on how to avoid readahead touching pages of neighbour block
3054e4a3    X not touching neighbour block works via setting MADV_RANDOM in last 1/4 of every block
18362227    X #5 access still triggers read to #4 ?
17dbf94e    X Provide mlock2 fallback for Ubuntu
d134c0b9    X wcfs: test: try to live with only hard memlock limit adjusted
c2423296    X Fix mlock2 build on Debian 8