Decrease the default shared memory size to 8GiB

Summary: Most likely 16GiB is not necessary for most users. Others can use the `"shared_memory": {"heap_size": <value>}` option in their `.pyre_configuration`.

Reviewed By: gbleaney

Differential Revision: D28546488

fbshipit-source-id: 3b9d06066928b6348439b219eb83d1aa2291c1e3

Author: arthaud

source/configuration.ml

@@ -14,7 +14,7 @@ let default_python_minor_version = 10
 
 let default_python_micro_version = 10
 
-let default_shared_memory_heap_size = 16 * 1024 * 1024 * 1024 (* 16 GiB *)
+let default_shared_memory_heap_size = 8 * 1024 * 1024 * 1024 (* 8 GiB *)
 
 let default_shared_memory_dependency_table_power = 27