vinyl: reset dump watermark after updating memory limit

The watermark is updated every second anyway, however not updating it
when the limit is reset results in vinyl/quota test failure:

 | --- vinyl/quota.result  Thu Mar 14 16:03:54 2019
 | +++ vinyl/quota.reject  Fri Mar 15 16:32:44 2019
 | @@ -146,7 +146,7 @@
 |  for i = 1, count do s:replace{i, pad} end -- does not trigger dump
 |  ---
 |  ...
 | -box.stat.vinyl().memory.level0 > count * pad:len()
 | +box.stat.vinyl().memory.level0 > count * pad:len() or box.stat.vinyl()
 |  ---
 |  - true
 |  ...

Closes #3864