Change behaviour of worker sharing.

A proxy worker is shared, if its URL is a leading substring
of the URL of another worker defined later in the configuration.

Before this change this
- triggered the "worker ... already used by another worker" message
- resulted in a hard to understand configuration for this worker
  - Attributes explicitely given in the later worker overwrote
    attributes in the worker which was defined first
  - General proxy attributes (e.g. ProxyTimeout ) overwrote
    attributes in the worker which was defined first

New behaviour:
- Talk about "sharing" of workers and log both worker URLs
  in the info log message instead of the "already used" message
- Do not overwrite attributes. If attributes were explicitely
  given in the later worker, log a warning that those are ignored
  because of sharing.

git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/trunk@987854 13f79535-47bb-0310-9956-ffa450edef68