Merge alfresco-solrclient into SearchServices

Merge the alfresco-solrclient project (with its history) into the SearchServices codebase. Archive the alfresco-solrclient project. We will only need to merge the histories for release branches after SearchServices release/V1.2.x (as before this it uses the alfresco-solrclient from the old repository project).

Update the existing git tags for alfresco-solrclient to have the format "alfresco-solrclient-6.0", so that we don't have a conflict between old versions of alfresco-solrclient and new versions of SearchServices.

In the future alfresco-solrclient should only be released as part of a release of SearchServiecs, and so we should update the version number of alfresco-solrclient to match the SearchServices version for each branch. Also we should consider updating the artifact id of alfresco-solrclient to something slightly different to prevent a conflict if we ever get to SearchServices 5.x (see https://artifacts.alfresco.com/nexus/#nexus-search;gav~~alfresco-solrclient~~~ )