Consider allowing configuration of buffer pool size

Currently the buffer pool size is hard-coded to be 80% of the process memory limit. We expect that this is a reasonable default but it may be useful to add a safety value command line argument to allow control over this.

E.g.
--buffer_pool_limit=80% would be the default, and it could be set to values like --buffer_pool_size=70% or --buffer_pool_size=40g