Reduce memory usage of SslHandler when OpenSslEngine is in use

Motivation:

JDK's SSLEngine.wrap() requires the output buffer to be always as large as MAX_ENCRYPTED_PACKET_LENGTH even if the input buffer contains small number of bytes.  Our OpenSslEngine implementation does not have such wasteful behaviot.

Modifications:

If the current SSLEngine is OpenSslEngine, allocate as much as only needed.

Result:

Less peak memory usage.