The goals of this task are to:

1) Improve our testing of the "returnKey()" feature in SBE a little so that we have better coverage for the kReturnKey req propagating to various buildXXX() methods in "sbe_stage_builder.cpp";

2) Identify the buildXXX() methods that  don't handle kReturnKey and don't assert that "reqs.has(ReturnKey)" is false, and for each method decide whether it should support kReturnKey or whether it should assert that "reqs.has(ReturnKey)" is false.