Memory management: Raise the default max memory usage for BigArrays to 50%.

The value had been chosen by taking into account the fact that eg. the filter
cache takes 10% and field data 60%. However, if you either don't need field
data or if you use doc values to store data on disk, then this would just leave
a lot of memory unused on your node. We will fortunately have a better story
for this once able to share memory accounting across several breakers.

Close #6375