Finalized BinaryTokenizer context handling. Polished.

Finalized BinaryTokenizer context handling. Polished.

No more funny context fields inside TLS structures. Context is handled
by the parsing code without needlessly storing it long-term.

Hid TLS structures/parsers used exclusively by
Security::HandshakeParser inside security/Handshake.cc to simplify API.

Also skipped unused ServerHello.random (instead of storing it in
TlsDetails::clientRandom) and replaced SQUID_TLS_RANDOM_SIZE macro
with a regular C++ constant.