driver-redpanda: set JVM memory limits based on host memory

[ligfx]

Fixes issues where the benchmark worker java processes take more memory than the client instance and keep getting hit by the OOM killer / causing SSH connections to get hit by the OOM killer.

Not sure if -Xmx50G should be made dynamic too?

[voutilad]

Not sure if -Xmx50G should be made dynamic too?

Yes. You're only setting the initial heap. You should be setting -Xmx as well. I think we should follow the practice of setting them to the same value so we don't have unexpected runtime heap growth which can impact the garbage collector. (Once upon a time there were optimizations it could do based on heap size...might still be the case, but it's been a long while since I dove into it.)