Remove connections list symbol from client environment at datashield.logout

[ymarcon]

Given that the list of DSConnection objects is discovered at runtime by the dsBaseClient functions (so that it is not specified in the functions parameters at each call), there can be some confusion when several lists of connections exists. Despite the error message recommends to disambiguate with datashield.connections_default, it would be more user friendly to remove the list of dead connections from the client environment after the logout has happened.

Example of output when confusion happens:

> ds.assign(connections, newobj="A", toAssign="c(0,1,2,3,4,5)")
  Assigned expr. (A <- c(0,1,2,3,4,5)) [=================================================] 100% / 1s
  Aggregated (exists("A")) [=============================================================] 100% / 1s
> ds.ls()
More than one list of DSConnection objects were found with no default specified: 'connections', 'conns'!
Error: Please specify a default list of DSConnection objects using datashield.connections_default()
> ls()
[1] "builder"     "connections" "conns"       "logindata"  
> datashield.connections_default("connections")
> ds.ls()
  Aggregated (lsDS(search.filter = NULL, 1L)) [==========================================] 100% / 1s