MEDIUM: ssl: Chain ckch instances in ca-file entries

MEDIUM: ssl: Chain ckch instances in ca-file entries

Each ca-file entry of the tree will now hold a list of the ckch
instances that use it so that we can iterate over them when updating the
ca-file via a cli command. Since the link between the SSL contexts and
the CA file tree entries is only built during the ssl_sock_prepare_ctx
function, which are called after all the ckch instances are created, we
need to add a little post processing after each ssl_sock_prepare_ctx
that builds the link between the corresponding ckch instance and CA file
tree entries.
In order to manage the ca-file and ca-verify-file options, any ckch
instance can be linked to multiple CA file tree entries and any CA file
entry can link multiple ckch instances. This is done thanks to a
dedicated list of ckch_inst references stored in the CA file tree
entries over which we can iterate (during an update for instance). We
avoid having one of those instances go stale by keeping a list of
references to those references in the instances.
When deleting a ckch_inst, we can then remove all the ckch_inst_link
instances that reference it, and when deleting a cafile_entry, we
iterate over the list of ckch_inst reference and clear the corresponding
entry in their own list of ckch_inst_link references.