compaction_controller: increase minimum shares to 50 (~5%) for small-data workloads
compaction_controller: increase minimum shares to 50 (~5%) for small-data workloads

The workload in #3844 has these characteristics:
 - very small data set size (a few gigabytes per shard)
 - large working set size (all the data, enough for high cache miss rate)
 - high overwrite rate (so a compaction results in 12X data reduction)

As a result, the compaction backlog controller assigns very few shares to
compaction (low data set size -> low backlog), so compaction proceeds very slowly.
Meanwhile, we have tons of cache misses, and each cache miss needs to read from a
large number of sstables (since compaction isn't progressing). The end result is
a high read amplification, and in this test, timeouts.

While we could declare that the scenario is very artificial, there are other
real-world scenarios that could trigger it. Consider a 100% write load
(population phase) followed by 100% read. Towards the end of the last compaction,
the backlog will drop more and more until compaction slows to a crawl, and until
it completes, all the data (for that compaction) will have to be read from its
input sstables, resulting in read amplification.

We should probably have read amplification affect the backlog, but for now the
simpler solution is to increase the minimum shares to 50 so that compaction
always makes forward progress. This will result in higher-than-needed compaction
bandwidth in some low write rate scenarios so we will see fluctuations in request
rate (what the controller was designed to avoid), but these fluctioations will be
limited to 5%.

Since the base class backlog_controller has a fixed (0, 0) point, remove it
and add it to derived classes (setting it to (0, 50) for compaction).

Fixes #3844 (or at least improves it).
Message-Id: <e7e69110db016cf9e61746fc45a9419ce6c19057@scylladb.com>

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