use SecureRandom for the multipart boundary and websocket key

[madib06ops]

computeMultipartBoundary draws the boundary from ThreadLocalRandom, which the JDK documents as not cryptographically secure: every value is mix32 over a per-thread 64-bit seed that advances by the fixed constant GAMMA, so an observer who recovers that state reproduces the whole sequence the thread will emit. Part content is never escaped, so the boundary is the only separator, and a caller that puts attacker-controlled bytes in any one field (a proxied upload, a user-supplied form value) lets that attacker close the part early and append a forged Content-Disposition, which the receiving server parses as an extra form field. getWebSocketKey builds the 16-byte Sec-WebSocket-Key nonce from the same generator, and RFC 6455 section 10.3 asks for a strong source of entropy there. Both now use a ThreadLocal, the same idiom already used for the Digest cnonce in Realm and the SCRAM nonce in ScramEngine.