MEDIUM: ssl: lookup and store in a ckch_node tree

MEDIUM: ssl: lookup and store in a ckch_node tree

Don't read a certificate file again if it was already stored in the
ckchn tree. It allows HAProxy to start more quickly if the same
certificate is used at different places in the configuration.

HAProxy lookup in the ssl_sock_load_cert() function, doing it at this
level allows to skip the reading of the certificate in the filesystem.

If the certificate is not found in the tree, we insert the ckch_node in
the tree once the certificate is read on the filesystem, the filename or
the bundle name is used as the key.