vinyl: use separate thread pools for dump and compaction tasks

Using the same thread pool for both dump and compaction tasks makes
estimation of dump bandwidth unstable. For instance, if we have four
worker threads, then the observed dump bandwidth may vary from X if
there's high compaction demand and all worker threads tend to be busy
with compaction tasks to 4 * X if there's no compaction demand. As a
result, we can overestimate the dump bandwidth and trigger dump when
it's too late, which will result in hitting the limit before dump is
complete and hence stalling write transactions, which is unacceptable.

To avoid that, let's separate thread pools used for dump and compaction
tasks. Since LSM tree based design typically implies high levels of
write amplification, let's allocate 1/4th of all threads for dump tasks
and use the rest exclusively for compaction.