With some LDAP server implementations, one server might provide
different "views" of the identites on different ports. One example is
the Active Directory Global catalog. The provider would contact
different view depending on which operation it is performing and against
which SSSD domain.

At the same time, these views run on the same server, which means the same
server options, enumeration, cleanup or Kerberos service should be used.
So instead of using several different failover ports or several
instances of sdap_id_ctx, this patch introduces a new "struct
sdap_id_conn_ctx" that contains the connection cache to the particular
view and an instance of "struct sdap_options" that contains the URI.

No functional changes are present in this patch, currently all providers
use a single connection. Multiple connections will be used later in the
upcoming patches.

In preparation for making struct be_req opaque.

In preparation for making be_req opaque

The sysdb context is already available through the 'domain' structure.

https://fedorahosted.org/sssd/ticket/1736

When there are no rules during first refresh, we don't have valid
USN value. We use 0 in this case, but it turned out that OpenLDAP
takes it as invalid time format (if modifyTimestamp is used instead
of USN) and thus returns no records.

Now we don't include USN/modifyTimestamp attribute in the filter
if such situasion occurs.

switch statements should always have a default section. In this
particular case gcc gave a "'send_fn' may be used uninitialized in this
function" warning.

https://fedorahosted.org/sssd/ticket/1689

Add a online callback if the first full refresh fails due to the
provider beeing offline so we can perform the refresh as soon as
possible.