BUG/MINOR: ssl: fix management of the cache where forged certificates are stored

BUG/MINOR: ssl: fix management of the cache where forged certificates are stored

First, the LRU cache must be initialized after the configuration parsing to
correctly set its size.
Next, the function 'ssl_sock_set_generated_cert' returns -1 when an error occurs
(0 if success). In that case, the caller is responsible to free the memory
allocated for the certificate.
Finally, when a SSL certificate is generated by HAProxy but cannot be inserted
in the cache, it must be freed when the SSL connection is closed. This happens
when 'tune.ssl.ssl-ctx-cache-size' is set to 0.