TLS handshaking issues with Postgres

[mpilquist]

Followup to #1896.

• Update test for completed handshake to not use isValid on SSLSession. On JDK 1.8.0_192, after the handshake completes with postgres, engine.getSession.isValid returns false and engine.getSession.getId returns an empty array. Same program works fine on JDK 11. This was tricking TLSEngine.read in to thinking it needed to handshake again, ad infinitum. Switched to comparing cipher suite to SSL_NULL_WITH_NULL_NULL, which is the documented cipher suite prior to handshake completion.
• Respect maxBytes on TLSSocket.read. This is needed b/c the sample program makes the correct assumption that calling socket.read(n) will return a chunk with <= n bytes, but TLSEngine was violating that.

Most of the diff is just passing maxBytes through the various calls.

Tested this on both JDK 1.8 and 11 using Rob's sample Postgres client.

[tpolecat]

SSL_NULL_WITH_NULL_NULL 👍 👍