memory-based Procstar agent load balancing

[alexhsamuel]

• agents report system stats (mem free, cache mem) to server
• server tracks recent system stats per connection
• load balancer uses this to allocate probabilistically based on (free + cache)
• global Apsis config for which load balancer to use and how to configure it

The load balancer should have access to:

• connection info, incl conn ID, user, hostname, so it can conditionalize
• recent system stats (incl age) per connection