[psp] Improve performance when searching for members by specifying source.

As of version 2.1.1, to improve performance, the source used when finding members (subjects) may be configured via a member source filter. Previously, all sources were searched, including the grouper source adapter, which is probably unnecessary.

The following

<!-- Return members from the "ldap" source only. -->
<grouper:Filter
xsi:type="grouper:MemberSource"
sourceId="ldap" />

may be added to a MemberDataConnector.

For example :

<!-- The MemberDataConnector returns attributes representing the member whose subject id or identifier is the principal
name. -->
<resolver:DataConnector
id="MemberDataConnector"
xsi:type="grouper:MemberDataConnector">
<!-- Return members from the "ldap" source only. -->
<grouper:Filter
xsi:type="grouper:MemberSource"
sourceId="ldap" />
<!-- Return the "dn" attribute of members whose subject source id is "ldap". -->
<grouper:Attribute
id="dn"
source="ldap" />
</resolver:DataConnector>