billing: Add bloom filter indexer and search tool

Billing files are a valuable tool for forensic analysis. It's good practice
to keep them indefinitly. Unfortunately, billing files may grow large, making
it necessary to compress them. Compressed billing files in turn are very
expensive to search, making analysis more difficult.

This patch introduces a tool set for building a bloom filter for billing files.
The bloom filter can be used as a compact and fast probabilistic index. The
indexes are not exact, having a small false positive rate. However as an index
this only means that we sometimes have to search a billing file in vein, not
that we miss any entries.

The /usr/sbin/dcache-billing-indexer tool is used to index and compress
billing files. For the Debian package, I added a cron.daily file that invokes
this script with the -yesterday -compress options. This means that for Debian
packages we compress and index yesterdays billing file. I don't have the OS
to test an RPM build and I suggest somebody else adds a similar file for RPM
based distros. If no billing files are found (like on a non-billing host),
nothing happens. The tool has an -all option to (re)index all billing files.

The /usr/bin/dcache script is extended with a single new command to search
billing files, using the index if available.

Target: trunk
Require-notes: yes
Require-book: yes
Acked-by: Tigran Mkrtchyan <tigran.mkrtchyan@desy.de>
Patch: http://rb.dcache.org/r/6075/