Fix: Rework SSL poll event handling to avoid infinite loops

The downstream connection callback must only be invoked when the event
that SSL requests for the connection to make progress has actually
occured. Otherwise, the downstream callback might do nothing but
re-queue an unrelated event (e.g. in user_net_io_want_write), and the
event loop comes around instantly while making no progress. Track the
SSL-requested events separately and deliver the required downstream
event when they fire.

Sample strace:

epoll_wait(0, {{EPOLLIN, {u32=96, u64=96}}}, 91, 10000) = 1
: net_ssl_callback in state tls_st_need_write calls cb NET_EVENT_WRITE
: User writes data, OpenSSL tries to write data
write(96, <snip>..., 170) = -1 EAGAIN (Resource temporarily unavailable)
: handle_openssl_error requests NET_EVENT_WRITE
epoll_ctl(0, EPOLL_CTL_MOD, 96, {EPOLLOUT, {u32=96, u64=96}}) = 0
: User callback then requests NET_EVENT_READ|NET_EVENT_WRITE
epoll_ctl(0, EPOLL_CTL_MOD, 96, {EPOLLIN|EPOLLOUT, {u32=96, u64=96}}) =
: Data available for *reading*
epoll_wait(0, {{EPOLLIN, {u32=96, u64=96}}}, 91, 10000) = 1
: net_ssl_callback in state tls_st_need_write calls cb NET_EVENT_WRITE
: again...