Upgrade Client::system_info

[texodus]

• Adds new fields to the struct returned by Client::system_info
  • used_size counts the used bytes if the allocator supports it
  • client_heap counts the max Client heap size.
  • client_used counts the used Client heap size.
  • cpu_time returns the cumulative milliseconds spent processing requests on the Server
  • cpu_time_epoch returns the cumulative milliseconds since cpu_time was reset.
• Adds a new demo dataset which showcases Client::system_info as a time series.
• Adds new features talc-allocator and trace-allocator which enable memory-tracing allocators to populate said fields (not default).
• Fixes a bug in perspective-workspace which caused the wrong viewer to be duplicated from the context menu.
• Fixes a bug in the engine which caused the index() ExprTK function to fail in conjunction with Table::update.