BUG/MEDIUM: ssl: properly remove the TASK_HEAVY flag at end of handshake

BUG/MEDIUM: ssl: properly remove the TASK_HEAVY flag at end of handshake

Emeric found that SSL+keepalive traffic had dropped quite a bit in the
recent changes, which could be bisected to recent commit 9205ab31d
("MINOR: ssl: mark the SSL handshake tasklet as heavy"). Indeed, a
first incarnation of this commit made use of the TASK_SELF_WAKING
flag but the last version directly used TASK_HEAVY, but it would still
continue to remove the already absent TASK_SELF_WAKING one instead of
TASK_HEAVY. As such, the SSL traffic remained processed with low
granularity.

No backport is needed as this is only 2.4.