Rename BBlockBufferCache and friends to *Pool*

Also move BBlockBufferPoolNoLock into BHPKG namespace with the other
classes. Not sure why it wasn't there before.
Some comment updates
Some *DataReader improvements

* BFDDataReader: Add SetFD().
* BBufferDataReader: Derive from BAbstractBufferedDataReader.
hpkg format: compress the whole heap

Instead of handling compression for individual file/attribute data we
do now compress the whole heap where they are stored. This
significantly improves compression ratios. We still divide the
uncompressed data into 64 KiB chunks and use a chunk offset array for
the compressed chunks to allow for quick random access without too much
overhead. The tradeoff is a limited possible compression ratio -- i.e.
we won't be as good as tar.gz (though surprisingly with my test
archives we did better than zip).

The other package file sections (package attributes and TOC) are no
longer compressed individually. Their uncompressed data are simply
pushed onto the heap where the usual compression strategy applies. To
simplify things the repository format has been changed in the same
manner although it doesn't otherwise use the heap, since it only stores
meta data.

Due to the data compression having been exposed in public and private
API, this change touches a lot of package kit using code, including
packagefs and the boot loader packagefs support. The latter two haven't
been tested yet. Moreover packagefs needs a new kind of cache so we
avoid re-reading the same heap chunk for two different data items it
contains.