TlsProgress.DONE when complete, NEED_READ or
NEED_WRITE when I/O is required, ERROR on failure.
TlsProgress handshake()Performs or continues the TLS handshake.
Call repeatedly until TlsProgress.DONE is returned. On NEED_READ, wait for transport readability and call again. On NEED_WRITE, wait for transport writability and call again.
TlsProgress.DONE when complete, NEED_READ or
NEED_WRITE when I/O is required, ERROR on failure.