[MRM-1097] for deployments/proxied artifacts, create and close indexing context when...

[MRM-1097] for deployments/proxied artifacts, create and close indexing context when indexing task is executed as compared to the original behavior where the indexing context is created before it is queued (so as the indexing task queue builds up, more indexing contexts are being opened/created which might be causing the too many open files problem)
Updated unit tests with changes made
Did not update the indexer as it would impact the API more significantly
Merged from: r1039004

git-svn-id: https://svn.apache.org/repos/asf/archiva/branches/archiva-1.3.x@1041824 13f79535-47bb-0310-9956-ffa450edef68