BUG/MEDIUM: ssl: Revamp the way early data are handled.

BUG/MEDIUM: ssl: Revamp the way early data are handled.

Instead of attempting to read the early data only when the upper layer asks
for data, allocate a temporary buffer, stored in the ssl_sock_ctx, and put
all the early data in there. Requiring that the upper layer takes care of it
means that if for some reason the upper layer wants to emit data before it
has totally read the early data, we will be stuck forever.

This should be backported to 2.1 and 2.0.
This may fix github issue #411.