[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]
addressbook
basket
blog
book
calendar
comment
company
email
file
folder
i18ncontent
keyword
link
mailinglist
person
tag
timeseries
vcsfile