MUST respond with 414 (URI Too Long) when request target exceeds limits.

MUST respond with 414 (URI Too Long) when request target exceeds limits.

Before the fix, Squid simply closed client connection after receiving a
huge URI (or a huge request-line), violating the RFC 7230 MUST. This
happened because a high-level Must(have buffer space) check in
ConnStateData::clientParseRequests() would throw an exception. Now these
problems are detected inside the low-level RequestParser code, where we
can distinguish huge URIs from huge methods.