vinyl: keep track of compaction queue length

Currently, there's no way to figure out whether compaction keeps up
with dumps or not while this is essential for implementing transaction
throttling. This patch adds a metric that is supposed to help answer
this question. This is the compaction queue size. It is calculated per
range and per LSM tree as the total size of slices awaiting compaction.
We update the metric along with the compaction priority of a range, in
vy_range_update_compact_priority(), and account it to an LSM tree in
vy_lsm_acct_range(). For now, the new metric is reported only on per
index basis, in index.stat() under disk.compact.queue.