Add a really fast API to fetch a custom bool setting from config

Summary: Add a builtin API to return a custom bool setting from
config.hdf. Since this value is constant during the lifetime of the hhvm
process, we can replace the call with the actual value at JIT time.
Note that I'm planning on adding server_get_custom_int_setting and
server_get_custom_string_setting in a follow up diff.

Reviewed By: @jdelong

Differential Revision: D1358762