Fix time_t/bigtime_t mixup.

Thanks stippi for noticing!
Fix concurrency issues in BSecureSocket

* Use pthread_once to initialize the SSL context once, in a thread-safe
way.
* Do not delete the BIO immediately when closing a connexion, instead
delay this to the destructor. This makes sure the protocol loop is done
running when we do that.
* Instead of creating a new BIO when we reconnect an already used
connection, create the BIO upfront, and reuse it with the new file
descriptor.
* Fix a memory leak: the SSL struct from OpenSSL was never freed, only
the BIO was.

Fixes #10414.