box: add internal box_on_select trigger

The trigger is invoked on min/max/get/select/iterator. The trigger
callback is passed a context with space, index, iterator type, and key
(MsgPack array). The trigger is run only if access and sanity checks
have passed. It will be used for auditing SELECT events in the EE
version.

NO_TEST=ee
NO_DOC=internal
NO_CHANGELOG=internal