This commit should fix#225.
In order to avoid stalling LDAP connections, Authelia creates new
sessions for each set of queries bound to one authentication, i.e.,
one session for authentication, emails retrieval and groups
retrieval.
Before this commit, a failing query was preventing the session to
be closed (unbind was not called). Now, unbind is always called
whatever the outcome of the query.
I took the opportunity of this commit to refactor LDAP client in
order to prepare the work on users database stored in a file.
(#233)
