vinyl: be pessimistic about write rate when setting dump watermark

We set the dump watermark using the following formula

    limit - watermark     watermark
    ---------------- = --------------
       write_rate      dump_bandwidth

This ensures that by the time we run out of memory quota, memory
dump will have been completed and we'll be able to proceed. Here
the write_rate is the expected rate at which the workload will
write to the database while the dump is in progress. Once the dump
is started, we throttle the workload in case it exceeds this rate.

Currently, we estimate the write rate as a moving average observed
for the last 5 seconds. This performs poorly unless the workload
write rate is perfectly stable: if the 5 second average turns out to
be even slightly less than the max rate, the workload may experience
long stalls during memory dump.

To avoid that let's use the max write rate multiplied by 1.5 instead
of the average when setting the watermark. This means that we will
start dump earlier than we probably could, but at the same time this
will tolerate write rate fluctuations thus minimizing the probability
of stalls.

Closes #4166