For DTLS, write()/read()/recv_into() called do_handshake() on every
call. do_handshake() runs wolfSSL_accept/connect, which on a
non-blocking socket can raise SSLWantReadError and abort an I/O long
after the handshake finished, and made DTLS write-side behaviour
inconsistent with TCP. Track completion with a _handshake_complete
flag set on a successful do_handshake(), and only drive the handshake
from I/O methods while that flag is False.
Kareem
Juliusz Sosinowicz