be_openssl: get rid off hackish "fd_is_set", to fix some corner cases

be_openssl: get rid off hackish "fd_is_set", to fix some corner cases

This patch is a cleanup and a bug fix, it drops ```fd_is_set``` flag, and
replace it with some checks to event_initialized(), and now we will not call
event_assign() on already added event, plus we will delete event when we really
have to (this patch fixes the case when server is down, IOW before this patch
we will not call event_del() because ```fd_is_set``` was reset to 0) and this
will fix some issues with retries in http layer for ssl.

Reported-in: #258
Fixes: regress ssl/bufferevent_socketpair_timeout
Fixes: regress ssl/bufferevent_socketpair_timeout_freed_fd