*don't use concept of reader-writer-body (which is broken), use
writer-body instead -> seval-handler now works correctly
*use new interface READ-MESSAGE-BODY from handler-lib for reading in the
message body
*rename READ-REQUEST-SEXP to PARSE-REQUEST-SEXP
*catch errors thrown by READ in PARSE-REQUEST-SEXP to answer 400
instead of 500 for requests whose message body doesn't contain a valid
s-expression