[multi-sources-removal] Drop cnxset_freed from source interface Though a hack is still needed when using sqlite Related to #2919300

[cnxset] add backwards compat for cnxset.source_cnxs It's used at least in a vcsfile migration script.

[multi-sources-removal] Simplify ConnectionsSet internal structures and public methods since it now handles a connection to the system source only Related to #2919300 [jcr: adjust 3.17.11 migration, fix a number of bugs in new ConnectionsSet implementation, fix source.{open,close}_source_connections]

[ldap] simplify connection handling since we deleted ldapuser source, we don't need anymore the get_connection and ConnectionWrapper stuff (that was used to put the ldap connection into the cnxset). Also, we should simply let connection errors propagate so this is properly reported to import logs.

[ldap] merge cw.server.ldaputils back into ldapfeed source we don't need separated modules since ldapuser source has been removed

[config] cleanup/refactor server sources file values handling * kill former `sources` method, misnamed since we've only the system source configuration in the sources file (and some default admin account information) * introduce new system_source_config and default_admin_config properties to access to the two information in this file Then use one or the other when appropriate

[repoapi] drop get_option_value's foreid argument repoapi was introduced in 3.19, so no need to introduce legacy deprecated stuff at the same time.

[multi-sources-removal] Drop foreid argument of repo.get_option_value It doesn't need it anymore. Related to #2919300