Fix json reader stall on http body trailer.

[evoskuil]

A completed json body reader returned zero bytes consumed without an error when beast re-fed the remainder of a Content-Length body, so http::async_read re-posted the same bytes forever. The thread never idled, pinning a pool thread per request and blocking threadpool::join on shutdown. The body is read before dispatch, so this is reachable before authorization with any application/json body whose trailing whitespace arrives in a separate read (Fulcrum sends \r\n inside Content-Length), or with any trailing non-whitespace.

• rpc reader: a non-terminated singleton (http) consumes trailing whitespace; non-whitespace after completion is jsonrpc_reader_stall when re-fed, otherwise the start of the next stream message.
• json reader: never writes a completed parser; trailing whitespace is consumed and anything else is extra_data.
• is_json_whitespace moved to json_body.hpp and shared.

Tests added for both readers. Syntax-checked locally only; not run.

[eynhaender]

fix confirmed working