Cache updating is done now by first reserving space where to write, and then

Cache updating is done now by first reserving space where to write, and then
writing to it whenever buffer gets full. There is no persistent cache file
locks anymore because of this, but it also means that the same cached field
may be written multiple times to the file by different processes. Also since
we reserve more space than we actually need at first, it some space can be
wasted if multiple processes are updating the cache.

--HG--
branch : HEAD