Add support for control of the timeouts for XrdAdaptor.
Be more aggressive about retrying on an invalid response.
Wait until any outstanding file-opens finish before deleting underlying
resources.

Additionally, make a number of improvements in the initial file-open
and error messages in the event of a failure.
Improve the retry handler for file-opens.
Merge pull request #3491 from bbockelm/multisource_xrootd_v4

xrootd -- Multisource xrootd v4