memtable controller: reduce adjustment period to 50ms
memtable controller: reduce adjustment period to 50ms

250ms is too high of a period for memtable controller. Since memtable
flushes are relatively efficient, specially in comparison to
compactions, if the shares are high we can flush a lot of data down with
the high shares - so in the next adjustment period our shares will be
minuscule and we won't flush much at all.

This leads to oscillating behavior that is mitigated by adjusting
faster.

Signed-off-by: Glauber Costa <7ba9037b0dd006f9408bd45b94707becd5cd4e87@scylladb.com>
Message-Id: <20180508182746.28310-3-7ba9037b0dd006f9408bd45b94707becd5cd4e87@scylladb.com>

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