vinyl: add global disk stats

This patch adds some essential disk statistics that are already
collected and reported on per index basis to box.stat.vinyl().
The new statistics are shown under the 'disk' section and currently
include the following fields:

 - data: size of data stored on disk.
 - index: size of index stored on disk.
 - dump.in: size of dump input.
 - dump.out: size of dump output.
 - compact.in: size of compaction input.
 - compact.out: size of compaction output.
 - compact.queue: size of compaction queue.

All the counters are given in bytes without taking into account
disk compression. Dump/compaction in/out counters can be reset with
box.stat.reset().