After a lot of benchmarking replaced the old super-smart bbcopy()

After a lot of benchmarking replaced the old super-smart bbcopy()
by a much simpler solution based on the bdirect interface and inlined
the fast path. Surprisingly, the new version is faster under real load
(the explanation is very simple: we use very large buffers for the
indexer and hence the bbcopy optimizations triggered rarely) and it also
works on all fastbuf streams, not only file-based ones.

Also, made bdirect_* inline.